controversies on the placement of cretaceous-paleogene boundary and the k/p mass extinction of...

Controversies on the Placement of Cretaceous-Paleogene Boundary and the K/P MassExtinction of Planktonic ForaminiferaAuthor(s): Richard K. Olsson and Chengjie LiuReviewed work(s):Source: PALAIOS, Vol. 8, No. 2 (Apr., 1993), pp. 127-139Published by: SEPM Society for Sedimentary GeologyStable URL: http://www.jstor.org/stable/3515167 .

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RESEARCH REPORTS 127~~~~~~~~~~~~~~~~~~~~~~~_

Controversies on the Placement of Cretaceous-Paleogene Boundary and the

K/P Mass Extinction of Planktonic Foraminifera

RICHARD K. OLSSON and CHENGJIE LIU

Department of Geological Sciences, Rutgers University, New Brunswick, NJ 08903

PALAIOS, 1993, V. 8, p. 127-139

Examination of recently reported K/P boundary sections indicates that the placement of the K/P boundary is based on equivocal criteria and that the boundary as placed is not synchronous. The conclusion that the K/P boundary in several U.S. Gulf Coast sections is complete and within a condensed section is simply the artifact of delineating the K/P boundary on disparate paleontologic datum planes and preservational bias of the microfossil assemblages. The upper slope and outer shelf sections at El Kef, Gredero, and Agost as well as the DSDP Site 577 section are continuous. The non-separation of Zone PO at some deep sea sites is due to very low sedimentation rates and the short time interval of Zone PO.

All Late Cretaceous planktonic foraminiferal species in the boundary sections included in this study, except Guembelitria cretacea Cushman, Hedbergella monmouthensis (Olsson) and H. holmdelensis Olsson, became extinct at the end of the Late Cretaceous. Species population ratios and species ranges in six K/P boundary sequences of different stratigraphic settings indicate that all other Cretaceous species occurring in the lower Paleocene are reworked specimens. Consequently, the loss of planktonic foraminifera at the end of the Cretaceous is extremely heavy. The stepwise extinction of Keller (1988, 1989) and the "foreshadowed" Late Cretaceous extinction and prolonged Paleocene extinctions proposed by Brinkhuis and Zachariasse (1988) are due to equivocal placement of the K/P boundary and treatment of reworked Cretaceous species occurring in the lower Paleocene as survivor species. The terminal Cretaceous mass extinction is most likely a geologically instantaneous single event which eliminated many groups of organisms. As for planktonic foraminifera, 50 out of 53 species became extinct at the end of Cretaceous. Only 3 species survived the mass extinction

event and were the stem forms for the subsequent radiation of all Paleocene planktonic foraminifera.

INTRODUCTION

Recent studies on the completeness of the Cretaceous- Paleogene (K/P) boundary in different stratigraphic sections and on the survivorship of Late Cretaceous planktonic foraminifera during the K/P transition have reached disparate conclusions. Some studies indicate that the K/P boundary in the Gulf Coastal Plain of Alabama and Texas is marked by an unconformity (Mancini et al., 1989; Olsson and Liu, 1990, 1991; Montgomery et al., 1992), whereas, other studies (e.g., Jones et al., 1987; Donovan et al., 1988; Keller, 1989) have proposed that the K/P boundary in Alabama and Texas lies within a continuous stratigraphic interval. Based on graphic correlation, MacLeod and Kel- ler (1991a, b) suggested that all deep sea sections have a hiatus straddling the K/P boundary in contrast to many continuous continental shelf sections.

The tempo and mode of the K/P mass extinction is even in greater disagreement. In his studies at El Kef and Gred- ero, Smit (1982) believed that Guembelitria cretacea was the sole Cretaceous species that survived into the Paleo- cene and, in turn, gave rise to all Paleocene planktonic foraminifera. Keller (1988) identified 10 species that survived into the Paleocene and proposed a stepwise pattern for the K/P mass extinction event at El Kef. She (1989) and her co-workers (Canudo et al., 1991) reached similar conclusions for the extinction pattern of Late Cretaceous planktonic foraminifera at Brazos River, Texas and Gred- ero and Agost, Spain. Brinkhuis and Zachariasse (1988) proposed a late Maastrichtian foreshadowed extinction of planktonic foraminifera followed by accelerated and prolonged early Paleocene extinctions at El Kef. From the

Copyright ? 1993, SEPM (Society for Sedimentary Geology)

RESEARCH REPORTS 127

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OLSSON & LIU

same sections (El Kef, Gredero), these workers recognized contrasting mortality and survivorship. The extinction patterns and survivorship of planktonic foraminifera in these studies may have resulted from the equivocal placement of the K/P boundary in some sections and/or treating the specimens of Cretaceous species occurring in the lower Paleocene in different ways. All workers agree that some Late Cretaceous microfossils are reworked into the lower Paleocene. They disagree on the magnitude of reworking and the exact number of Cretaceous species affected by reworking. Treating all Cretaceous species, except Guem- belitria cretacea, that occur in the lower Paleocene as reworked specimens, Smit (1982) suggested a single catastrophic event that eliminated all planktonic foraminifera but one species. Recognizing some specimens of Creta- ceous species as indigenous species, other workers (Keller, 1988, 1989; Brinkhuis and Zachariasse, 1988; Barrera and Keller, 1990; Canudo et al., 1991) proposed a pattern of stepwise extinctions of planktonic foraminifera that started in the Late Cretaceous and lasted into the early Paleo- cene. Bryan and Jones (1989) observed an earlier macrofaunal extinction and a later nannofloral extinction at Braggs, Alabama. Since these observations were made by different workers and based on sections deposited in different paleoenvironments, disparate stratigraphic schemes may have played a role in their interpretations. In order to investigate the real pattern as well as the cause of the K/P boundary mass extinction and to eliminate artifactual effects, we here first clarify the definition of the K/P boundary, then discuss several individual K/P boundary sections, and finally approach the mass extinction issue with an unequivocal stratigraphic background.

BIOSTRATIGRAPHY OF THE K/P BOUNDARY SECTIONS

Delineation of the K/P Boundary and Lower Paleocene Planktonic Foraminiferal Biostratigraphy

In November 1988, members of the Paleogene Working Group on the Cretaceous-Paleogene boundary under the International Commission of Stratigraphy of IUGS voted to designate a stratotype for the K/P boundary. They voted to place the boundary at the base of the black clay layer at El Kef, Tunisia to be the GSSP (Global Stratotype Section and Point) (Cowie et al., 1989). In the El Kef section, both the upper Maastrichtian and lower Danian consist of white-grey marls which are separated by an approximately 50 cm thick black clay layer (Keller, 1988). Since the base of the black clay is a lithologic boundary, lithostratigraphic correlation of the K/P boundary on a global scale is problematic because of the restriction of the black clay lithofacies. Due to this restriction, efforts have been made to correlate strata by chronologic, magnetic or paleontologic means in lieu of lithologic correlation. The most frequently used is paleontologic correlation.

On paleontologic criterion, the K/P boundary is placed either at the level of mass disappearances of Late Creta- ceous species or at the first occurrence (FO) of Paleocene

species. These equivocal definitions of the K/P boundary have frequently caused a problem, because, in expanded continental shelf sections, the FO of Paleocene species is higher than the level of mass extinction of Cretaceous species (Smit, 1982; Jones et al., 1987; Keller, 1988; Olsson and Liu, 1990; Liu and Olsson, 1992). In the El Kef section, the base of the Boundary Clay coincides with an abrupt decline in abundance of Late Cretaceous planktonic foraminifera and a significant increase in the survivor species, Guembelitria cretacea Cushman (Liu and Olsson, 1992). The FO of Paleocene species (Parvularugoglobigeri- na aff. eugubina = Globigerina minutula of Smith, 1982, and Globoconusa conusa of Keller, 1988, 1989) is at 7 cm (Sample #542) above the K/P boundary (Liu and Olsson, 1992) and even higher by other observations (Smit, 1982; Keller, 1988; Brinkhuis and Zachariasse, 1988). Therefore, in a strict sense, the paleontologic K/P boundary is the mass extinction level of Late Cretaceous fauna. Although upward reworking of Cretaceous species is a potential problem as Keller (1989) pointed out, the concept of a "golden spike" which is at the level of the mass disappearance of Cretaceous species in the stratotype El Kef section, should be followed.

Upward reworking can be recognized by several carefully evaluated criteria. In addition, the FO of Paleocene species is at least equally troublesome because of downward reworking, the extreme rarity and poor preservation of the earliest Paleocene species in most of the K/P boundary sections. In any case, the FO of Paleocene species is an invalid criterion for defining the K/P boundary because no previous study has reported the FO of Paleocene species from the boundary layer in the stratotype El Kef section. We emphasize the extreme importance of an unequivocal definition of the K/P boundary because it shows that equivocal placement of the K/P boundary has caused tre- mendous chaos in studies related to the sea-level history, chronology of sequence boundaries, tempo and mode as well as the ultimate cause of the mass extinction event during the K/P transition. In order to avoid the possible effect of upward reworking of Cretaceous species on the placement of the K/P boundary in individual sections and to follow the GSSP at El Kef, Liu and Olsson (1992) utilized the increase in abundance of G. cretacea as the bio- chronologic K/P boundary. Since the initial abundance increase of this survivor species is caused by the extinction of other Cretaceous species, this datum plane is identical to the mass disappearance of Late Cretaceous species which is widely accepted as the K/P boundary (Berggren et al., 1985; Berggren and Miller, 1988). It also follows the K/P boundary GSSP recommended by the Paleogene Working Group.

In 1982, Smit erected Zone P0 (Guembelitria cretacea Zone) to designate an interval which spans from the abrupt decrease of Cretaceous planktonic foraminifera to the FO of Paleocene species at El Kef, Tunisia and Gredero, Spain. Several workers recently reported on the occurrence of Zone P0 in some shallow water sections in the U.S. Gulf Coast, Israel and Spain (Keller, 1989; D'Hondt and Keller, 1991; Olsson and Liu, 1990; Canudo et al., 1991). Zone P0

128 l



CRETACEOUS-PALEOGENE BOUNDARY AND PLANKTONIC FORAMINIFERA

COASTAL ONLAP LITHOLOGY FORMATION

This Study Donovan et al., 88' | Haq et al., 87', 88'

e TA 1.2 TST TA 1.2 TA 1.3 ,~

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FIGURE 1 -A comparison of the latest Cretaceous and earliest Pa- leogene eustatic cycle charts of Haq et al. (1987, 1988), Donovan et al. (1988) and this study based on the sequence stratigraphy and microfossil biostratigraphy in the Alabama Gulf Coast.

has not been identified unequivocally in deep sea sections. The top of Zone PO was originally defined as the FO of Parvularugoglobigerina aff. eugubina (=Globigerina minutula of Smit, 1982 and Globoconusa conusa of Keller, 1988,1989). Keller (1988) redefined Zone PO as the interval from the K/P boundary to the FO of Par. eugubina (Lu- terbacher and Premoli-Silva) and subdivided the Par. eugubina Zone into Subzones Pla and Plb. In the zonal scheme of Berggren and Miller (1988), the top of the Par. eugubina (Pa) Zone is placed at the FO of Parasubbotina pseudobulloides (Plummer). Our observation suggests that the LO of Par. eugubina is more abrupt and easily recognized than the FO of Ps. pseudobulloides. Therefore, the LO of Par. eugubina is used here as the top of Zone Pa (Par. eugubina). We accept the P-lettered zonal schemes of Berggren and Miller (1985) with the adoption of Smit's Zone PO. Thus, in this study, Zone PO (G. cretacea) is from the K/P boundary to the FO of Par. eugubina. Zone Pa (Par. eugubina) is the total range of the nominal species. Subzone Pla is from the LO of Par. eugubina to the FO of Subbotina triloculinoides (Plummer). All younger zones (subzones) are that of Berggren and Miller (1988).

The K/P Boundary Stratotype-El Kef Section

In the El Kef section, the uppermost Cretaceous contains rare individuals of Abathomphalus mayaroensis, the marker species for the uppermost Maastrichtian. The lowermost Paleocene contains Zone PO (Guembelitria cretacea). The Cretaceous-Paleogene transition is biostratigraphically continuous and there is no physical evidence of a disconformity near the K/P boundary in the El Kef section.

Preservation of planktonic foraminifera in the lower Da- nian at El Kef is poor due to severe dissolution but it

^_^_ -^1-^ ARCTIC

rs~~~~~~~~~~~~~ D< A R, N 356

5 ^ 1v2 (3..)9 ^\/^

3 3 577 7EE7 :7EfE7EEEi

.z.~ o-\ , PACIFIC

PACIFIC f I X i;

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FIGURE 2-Location map of the key K/P boundary sections in the Gulf Coast of Mexico, Mediterranean and deep seas.

becomes moderate upward and even good in some higher horizons. Because of this taphonomic effect Paleocene cancellate walled species show an irregular distribution and abundance. Cancellate species which are more susceptible to dissolution are less abundant and have later first occurrences (FO's) compared with other sections which contain better preserved assemblages. Zone PO at El Kef is 37 cm thick and Zone Pa (Par. eugubina) attains a thickness of 813 cm. The sedimentation rate increased by 355 % from 0.74 cm/kyr in Zone PO to 2.63 cm/kyr in Zone Pa using the chronological data of Berggren et al. (1985). The extremely low sedimentation rate in the earliest Paleocene may reflect the nearly entire shutdown of plankton productivity at the end of the Cretaceous (Zachos et al., 1989).

Braggs Section in Alabama

The Cretaceous-Paleocene section at Braggs has played a primary role in the interpretation of sequence stratigraphy at the K/P boundary. On the cycle chart of Haq et al. (1987, 1988) the K/P boundary is placed within a transgressive systems tract in cycle TA 1.1. The stratigraphic evidence for positioning the boundary within a transgressive systems tract is based on studies by the Exxon group on the Braggs section (Baum et al., 1988; Donovan et al., 1988). The boundary was placed at the top of bed 5 in the Clayton Formation based on the first recognition of Pa- leocene calcareous nannofossil species (Figs. 1, 2). Re- cently, Habib et al. (1992) have recovered Paleocene nannofossil species in bed 4 and placed the K/P boundary at the top of bed 2 (i.e., at the top of the Prairie Bluff Chalk). Mancini et al. (1989) used planktonic foraminiferal data to also place the boundary at the top of the Prairie Bluff Chalk.

Microfossil recovery in the basal beds of the Clayton Formation is poor due to cementation, diagenesis, and dissolution. Consequently, a clear biostratigraphic deter- mination of the K/P boundary has been hampered by the rarity of age diagnostic microfossils in these beds. For instance, bed 3 which rests upon the Prairie Bluff Chalk is an indurated calcareous sandstone which has not yielded

129



OLSSON & LIU

Littig Quarry, Brazos River, Millers Ferry, Braggs, Cm (Texas) (Texas) (Alabama) (Alabama) Legend

3~~00 - : . - - p w $

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Fm.

200 TSAPlabm ancemesentr of K/P placed at the sequence boundaries '' within the sBoundary in the

U.S. Gulf Coast.

Littig (D Manc ini et al., 1989; Member 6

f '

Clayton i Habib et al, 1991

onstrates -hBasal (2)/ a y ones et l., 1987.

100- whee Sands 3 Aaama

00-

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~(M Sandsii et al. ~ 1989;~~ Olsson & L0u, 990 .T... a 2 (3) Pessagro & Longona,

CA Kellr, 1989.

K/P boundary on the cycle)chart Jiang & Garmer ,

K - ---upermnost-0K : uppermost - , ...- Mastrichtian Maestrichtian Corsicana Fm. Prairie Bluff

FIGURE 3 -Lithostratigraphic correlation of K/P boundary sequences in Alabama and east-central Texas, showing that the K/P boundary in different sections or in the same section were previously equivocally placed at the sequence boundaries or within the basal Paleocene transgressive systems tract. Arrows point to different horizons of the "K/P boundary" by different workers.

identifiable calcareous nannofossils. However, we have been able to retrieve the Paleocene planktonic foraminiferal species Globoconusa daubjergensis from this bed along with a number of benthic foraminifera, although all are poorly preserved. Thus, the biostratigraphic data dem- onstrates that the K/P boundary at Braggs can be confi- dently placed at the top of the Prairie Bluff Chalk which is where the boundary is placed elsewhere in Alabama (Mancini et al., 1989; Olsson and Liu, 1990; Liu and Olsson, 1992).

This biostratigraphic revision at Braggs means that the K/P boundary on the cycle chart of Haq et al. (1987, 1988) coincides with the sequence boundary between cycles TA 1.1 and TA 1.2 and with the boundary separating the Prai- rie Bluff Chalk and the Clayton Formation (Fig. 1). How- ever, in the study of Donovan et al. (1988) at Braggs cycle TA 1.1 is changed to UZA 4.6 in the cycle chart and cycle TA 1.2 is changed to TA 1.1. This revision reflects their placement of the K/P boundary within the Clayton For- mation which is correlated with TA 1.2 on the chart of Haq et al. (1987, 1988). The change reflects their belief (which we believe is correct) that this is the first cycle of the Tertiary. Cycle TA 1.1 of Haq's version is renamed UZA 4.6 to designate it as a Cretaceous cycle. We place the Clayton Basal Sands, although considered by Donovan et al. (1988) as Cretaceous in age, within the lowstand deposits of their TA 1.1 along with the Prairie Bluff Chalk (Fig. 1).

Mancini et al. (1989) interpreted the hiatus of the unconformity at Braggs and elsewhere in Alabama and Mis- sissippi to extend from the middle Maastrichtian to Da- nian Zone P1. They reached this conclusion because the uppermost Maastrichtian planktonic foraminiferal index species Abathomphalus mayaroensis was not found in the Prairie Bluff Chalk and the lower Danian species Parvu- larugoglobigerina eugubina was not observed in the Clay- ton Basal Sands. More recent biostratigraphic data shows

that the top part of the Prairie Bluff Chalk at Braggs contains the latest Maestrichtian calcareous nannofossil species Micula prinsii (Habib et al., 1992). The occurrence of this species indicates that A. mayaroensis is absent because of paleoecologic reasons. It apparently was a more open ocean dweller and would have been excluded from the shallow shelf environment during deposition of the Prairie Bluff Chalk. Micula prinsii also occurs in the top part of the Prairie Bluff Chalk at Millers Ferry (Aubry, pers. comm., 1991; Olsson et al., 1992) where the overlying Clayton Basal Sands contain Zones P0 and Pa. Conse- quently, a hiatus coincident with the K/P boundary in Alabama and Mississippi, if present, must be of very short duration. The tempo and mode of K/P extinction in different fossil groups based on the Braggs section (Jones et al., 1987; Bryan and Jones, 1989; MacLeod and Keller, 1991a, b) must be reevaluated with the present K/P boundary assignment which is consistent with the stratotype at El Kef. Thus, the diachronous extinction of macrofossils and nannofossils concluded in the study of Bryan and Jones (1989) and the hiatuses based on graphic correlation by MacLeod and Keller (1991a, b) must also be reevaluated.

Millers Ferry (Alabama) Sequence

The Millers Ferry section is located approximately 35 miles west of Braggs (Fig. 2). Lithology and stratigraphic sequence at Millers Ferry are similar to that at Braggs. The Millers Ferry K/P sequence consists of the Upper Cretaceous Prairie Bluff Formation and the lower Paleo- cene Pine Barren Member of the Clayton Formation (Man- cini et al., 1989). The K/P boundary black clay layer is not present and the boundary is placed between the Prairie Bluff Chalk and the base of the Clayton Basal Sands (CBS) where Guembelitria cretacea begins to increase in relative abundance. The Prairie Bluff Chalk contains abundant microfossils and, according to Donovan et al. (1988), was deposited during a sea-level highstand. The Clayton Basal Sands, which they interpreted as a lowstand wedge (LSW) deposit, occurs at the base of the Paleocene and is followed upward by sediments of a transgressive systems tract (TST) and a highstand systems tract (HST) (Fig. 3).

An undulating surface separates the Upper Cretaceous and the lower Paleocene. This surface has been interpreted as a Type-I unconformity by Mancini et al. (1989). They concluded that the upper Maastrichtian is absent at this unconformity. Recently, the latest Cretaceous nannofossil Micula prinsii was identified in the uppermost part of the Prairie Bluff Chalk (Aubry, pers. comm., 1991; Olsson et al., 1992). The lower Paleocene Clayton Basal Sands contain the thickest Zone PO (59 cm) so far found (Olsson and Liu, 1990). Consequently, the hiatus separating the Clay- ton Basal Sands and the Prairie Bluff Formation, if present, is much shorter in duration than previously believed. As stated above, we place both the Prairie Bluff Chalk and the Clayton Basal sands within the lowstand systems tract of Cycle TA 1.1 (Fig. 1).

The Millers Ferry sequence contains excellently pre-

130




served planktonic foraminiferal assemblages although at certain horizons the preservation is moderate or even poor. The stratigraphic section ranges from the uppermost Maastrichtian to Subzone Pla (Parasubbotina pseudobulloides). Zone P0 is 59 cm thick and Zone Pa (Parvu- larugoglobigerina eugubina) 193 cm thick. The sedimentation rate is 1.18 cm/kyr for Zone P0 and 0.62 cm/kyr for Zone Pa respectively. These deceasing sedimentation rates contrast distinctly with the increasing rates at El Kef. It is perhaps due to sediment starvation on the continental shelf at Millers Ferry during sea-level rise whereas at El Kef carbonate supply on the outershelf increased significantly due to recovery of plankton productivity. Since the Millers Ferry sequence contains planktonic foraminiferal assemblages exhibiting sequential evolution, the interval from the K/P boundary to middle Subzone Pla is considered complete.

Mussel Creek Section in Alabama

The Mussel Creek K/P boundary section is located approximately 4 miles south of the Braggs section (Fig. 2). The Clayton Basal Sands that occur at Mussel Creek are about 80 cm thick. Samples collected from the basal 48 cm of the Clayton Basal Sands by Dr. Habib yield rare individuals of Guembelitria cretacea but do not contain Paleocene species. This interval should be correlated to Zone P0 at Millers Ferry. A fairly well preserved Zone Pa planktonic foraminiferal assemblage including species of Parasubbotina Olsson et al. (1992), Eoglobigerina, Prae- murica Olsson et al. (1992), Globanomalina, Woodringina and Globoconusa occurs at 48 cm above the base of the Clayton Basal Sands. Thus, the biostratigraphy at Mussel Creek is similar to that at Millers Ferry. The presence of Zone P0 at Mussel Creek and its absence at Braggs may be due to the depositional environment (Mussel Creek was more offshore than Braggs), or due to the undulating surface at the top of the Prairie Bluff Chalk.

Brazos River and Littig Quarry (Texas) Sections

Brazos River Sections

The K/P boundary sections exposed along Brazos River, Falls County, Texas have been intensively studied. At Brazos River, the placement of the K/P boundary is also equivocal, being drawn, on the one hand, at the unconformity separating the Corsicana Formation below and the "tsunami bed" above (Montgomery et al., 1992) and, on the other hand, near the base of the Kincaid Formation above the "tsunami bed" (Jiang and Gartner, 1986; Keller, 1989) (Fig. 3). Jiang and Gartner and Keller placed the K/P boundary within the Kincaid Formation based on the increase in the abundance of Paleocene nannofossils. How- ever, Montgomery et al. (1992) reported the presence of Danian planktonic foraminifera in the "tsunami bed" at Brazos River. Consequently, based on planktonic foraminifera, the K/P boundary should be placed between the top of the Corsicana Formation and the base of the "tsunami bed."

Littig Quarry Section

In the Littig Quarry section which is approximately 73 miles to the southwest of Brazos River (Fig. 2), Jiang and Gartner (1986) placed the K/P boundary at the top of Corsicana Formation and below a basal sand bed (Littig Member) of the Kincaid Formation on the abrupt decline of Cretaceous nannofossils and on the increase of Paleo- cene species. We have recovered a middle to late Zone Pa planktonic foraminiferal assemblage (Eoglobigerina, Glo- banomalina, Parasubbotina, Praemurica, Globoconusa, Woodringina and Guembelitria) from the basal 60 cm of the Littig Member at Littig Quarry.

At Brazos and the Littig Quarry the Kincaid Formation rests on an irregular scoured surface at the top of Corsicana Formation. In fact, the lithology and fossil content as well as stratigraphic setting of the "tsunami bed" and the Littig Member in Texas are similar to the Clayton Basal Sands in Alabama which are interpreted as lowstand deposits (Mancini et al., 1989; Olsson and Liu, 1990). The basal Paleocene beds in Texas probably have a similar origin and were deposited as lowstand sands. Since the Clayton Basal Sands contain a sequential biostratigraphy from the base to the top as well as a changing benthic paleoecology, they can not have been deposited by a single rapid event. Savrda (1991) on sedimentological criteria concluded that the Clayton Basal Sands were deposited in shallow marine and estuarine environments. Nevertheless, a tsunami event could have eroded the top of the Corsicana Formation and the Prairie Bluff Chalk. Alternately, a very short interval of sea-level fall could have occurred at the K/P boundary and, possibly, overlapped or was synchronous with a tsunami storm. In any case, sea level was at a lowstand in the Gulf of Mexico during the K/P transition.

K/P Boundary Section at DSDP Site 577, Shatsky Rise, N.W. Pacific

The Cretaceous-Paleocene sequence recovered at DSDP Site 577 on Shatsky Rise, Northwestern Pacific is considered complete by most workers (Heath et al., 1985; D'Hondt and Keller, 1991). However, MacLeod and Keller (1991a, b) recently suggested that the K/P transition at Sites 528 and 577 is marked by a hiatus based on graphic correlation of these two sections with continental shelf sections.

At Site 577, Zone P0-Pa interval is 42 cm, Subzone Pla 244 cm and Subzone Plb 330 cm, equaling a sedimentation rate of 0.116 cm/kyr, 0.154 cm/kyr and 0.220 cm/kyr respectively, indicating an increasing sedimentation rate during the early Danian. This is probably due to a productivity increase after a dramatic drop at the K/P transition (Zachos et al., 1989). Our analysis of sites 577 and site 528 shows that zone P0 is not separable.

Agost and Gredero Sections in Spain

The K/P boundary sections at Gredero (Caravaca) and Agost in Spain have been studied by Smit (1982) and Canudo et al. (1991). These two sections are continuous,

131



132 OLSSON & LIU

Millers Ferry, Braggs & DSDP 356 & Mussel Creek El Kef ODP 750A

Brazos River &Gredero Surface of LiBtig Quarry& Agost DSDP577 Erosion

Maestrichtian BC chalk & clay, I

CBS: Clayton Basal Sands Conformable TB: "Tsunami Bed" Surface BC: Boundary Clay

FIGURE 4-Stratigraphic settings of the K/P boundary section localities during the Cretaceous-Paleogene transition.

being similar to the El Kef section but perhaps deposited in a more oceanic environment (Fig. 4). The K/P boundary in both sections, as at El Kef, is placed at the base of the black clay layer. Preservation of planktonic foraminiferal assemblages is poor. From Smit's (1982) illustrations and our observation, the original test walls of planktonic foraminifera from the lower Danian, in many cases, were dissolved and only the internal casts are left. In many aspects, these Spanish sections are similar to the El Kef section.

CONTINUITY OF THE K/P BOUNDARY SECTIONS

As discussed above, the continuity of certain K/P boundary sections is controversial. The continuity of continental shelf sections depends on where the K/P boundary is placed. The continuity of pelagic sections depends on how one interprets the direct contact between Zone Pa and the Upper Cretaceous. Utilizing Shaw's (1964) graphic correlation method, MacLeod and Keller (1991a, b) concluded that all deep sea sections have a hiatus at the K/P transition while the upper slope and continental shelf sections are continuous.

Graphic correlation is a technique developed by Shaw (1964) that utilizes the total range of fossils to achieve more reliable time-stratigraphic control between stratigraphic sections. The technique has been expanded to include unambiguous geophysical and geochemical stratigraphic information (Hazel et al., 1984; Prell et al., 1986; Edwards, 1984). In order to achieve a high degree of ac- curacy graphic correlation relies on a data base of total ranges of taxa which is expressed as first occurrences (FO) and last occurrences (LO). The FO or the LO of a taxon which ranges above or below a stratigraphic section can be included in the data base but, however, the ultimate position of an occurrence is based on the total range data. The logic of the graphic correlation technique is to locate time equivalent data points in stratigraphic sections. The technique involves an iterative comparison of range data between stratigraphic sections in order to determine where

the most probable positions of data points (FO, LO, geophysical and geochemical levels) would be in a standard section (termed the composite reference section). The technique begins with the comparison of two sections, one of which is designated the standard reference section. Taxa range data from the two sections are plotted on different axes on a conventional two-axis graph and a visual correlation is expressed as a Line of Correlation (LOC). The LOC then makes it possible to transfer to the "selected" standard reference section data from the two compared sections which express the best information on the total range of taxa. Additional sections are compared to the standard reference section which accumulates additional information on ranges of taxa. The end result of repeated exercises is the construction of a composite reference section which presumably includes all the lowest FO's and highest LO's. The objective of the iterative comparison of range data from stratigraphic sections is to approximate the total range of taxa and, thereby, to improve stratigraphic correlation.

As long as the relative rates of sedimentation are more or less constant (which appears to be the normal situation) the LOC is a straight line given the condition that FO's or LO's of the same taxon are nearly time equivalent in different sections. Changes in rates of sedimentation are expressed by a sharp bend in the LOC. A hiatus in a section being compared to the composite reference section is expressed graphically as a line segment parallel to the axis of the composite reference section.

In their application of the graphic correlation technique to identify hiatuses in K/P boundary sections MacLeod and Keller (1991b) used 28 FO's and 47 LO's of taxa (main- ly planktonic foraminifera) in the construction of a composite reference section. A breakdown of this data shows that it can be categorized as three sets of data; 41 taxa are represented by LO but no FO data, 22 taxa are represented only by FO data, and 6 taxa have FO and LO data. The LO data set contains exclusively Cretaceous taxa and the FO data set exclusively Paleocene taxa. This is a rather unusual situation in that so little total range data are used in applying the graphic correlation technique.

Of the 41 Cretaceous taxa used by MacLeod and Keller 26 have an LO in the Paleocene. These taxa are regarded by them as Cretaceous survivors. Most workers (Smit, 1982; Olsson and Liu, 1991; Pessagno and Longoria, 1990; among others) regard almost all Cretaceous planktonic foraminifera that occur in the basal Paleocene as reworked. Nev- ertheless, if, indeed, the 26 Cretaceous planktonic taxa used in graphic correlation by MacLeod and Keller did survive into the Paleocene, they should have consistent last occurrences that will be evident on the composite reference section. MacLeod and Keller presumably used the highest LO where each of the Cretaceous taxa were identified in the stratigraphic sections that were utilized. We have compared the Paleocene occurrences of most of the Cretaceous taxa used by them in graphic correlation with their occurrences at Millers Ferry (Fig. 6). It can be observed in Figure 6 that there is no consistent level at which the LO of a taxon is observed. The only Cretaceous taxon




EL KEF MILLERS FERRY

Cm Cm

DSDP Site 577

Cm

FIGURE 5-A conventional stratigraphic correlation of FO's in 3 K/P boundary sections to show preservational and depositional effects on paleontological data (data from Table 1).

that does have a consistent occurrence in the basal Paleo- cene is Guembelitria cretacea which is a definite Creta- ceous survivor (Smit, 1982). The highest LO of any taxon invariable is in the Millers Ferry section with the last occurrences observed in the basal part of Zone Pla (Par- asubbotina pseudobulloides). The higher occurrences at Millers Ferry can be explained by a higher reworking potential in the shallow shelf environment in which the Mil- lers Ferry section was deposited. Many of the Cretaceous taxa that occur in the basal Paleocene are contained in clasts of Cretaceous Prairie Bluff Chalk that are obviously reworked. Even if it is assumed that some of the Cretaceous taxa did survive into the Paleocene, it is impossible to separate true range from reworked range. It must be concluded that the use of Cretaceous taxa by MacLeod and Keller in their graphic correlation exercise is not an ap- propriate data set.

There are significant problems with preservational bias in lower Danian sections (Olsson et al., 1992). Globigerine taxa in the lower Danian are very small and have very thin test walls which are susceptible to dissolution. Sections with cementation and poor preservation of planktonic foraminifera are suspect for preservational bias. The graphic correlation technique is designed to overcome preservational problems by the iterative comparison of stratigraphic sections so that such problems in one section could be overcome by better preservation in other sections. By this process the true range of a taxon can be best approximated. The FO data set of 22 Paleocene taxa used by MacLeod and Keller is faced with preservational problems. The taphonomic removal of foraminifera at Millers Ferry is not a serious problem. Borehole Core 225 at Millers Ferry contains the thickest Zone P0 (Guembelitria cretacea) and the best preserved microfossil assemblage so far found. Thin walled lower Danian taxa are abundant with little or no infilling of the tests. The details of the wall texture as viewed by SEM show the original features

Cretaceous Maestri- Zone PO Zone Pa Subzone Pla Taxa chtian (G. cretacea) (Par. eugubina) (Ps. pseudobulloides)

Guembelitria cretacea _ _

H. monmouthensis______ __ H. holmdeleusis ", Rosita contusa - ?

Racemig. fructicos. . .--

Pseudogu. costulatau_ _ _ _ _ __

Globotr. stuartiformis _

Plnogl. brazoensis ___ _ Globotr. cornca -- - - - - - - - -

Rugogl. hexacamerata

Globigerin. mulispina - -- - Globotrucana dwi- -- ---

Globotruncan. gansseri Globott. trinidadenwis - - - - Globotrncana arca ---- Globot. stephensoni Racemigu. powelli -------

Pseudogu. palpebra -- ; Rugoglobig. reicheli ---

Rugoglobig. rugosa - ---. -

Globig. subcarinatus_ __ __.

Globotr.falsostuarti - ... ... - - - c- - -

_- _ J

Rugogl. milamensis - - - -- . Glob. prairiehillensis - -- --- ---

Rugog. macrocephala ---- - - -...-

Globotr. aegyptiaca ---- -. - --................................

Globigerinel. aspera ' - -. .

Heterobel. cf. moremani ..-- - --- .- Planoglobul. carseyaeo -------- - .

Rug. subcircumnodfer..--.-------- - - .--....... Heterohelix glabraws - - - --

Glublerina acura _..-- .- - Pseudotex. deformis - ..............- ...... Pseudogu. excolata -----------....... Pseudoteat. elegans ...... ...... - .....- - .....

Globotruncanea citae --- ---------- -, Globotrunc. petaloidea - -. - - -. - - - -......

Heterohelix striata . - . -. . ....-. - -- - -

Rugoglobigerina scotti .... ................~.

...

, ~

Heteroh. navarroensis .................... .. . - . .. ... ... ... . ... ......

Heterohelix globulosa . .......

Millers Ferry _ - - -- El Kef

Brazos-l

..... -Brazos Core Caravaca

-- -- Agost

FIGURE 6-Occurrences of 41 Late Cretaceous planktonic foraminiferal species in the lower Paleocene in 6 K/P boundary sections of different stratigraphic settings. Data sets are based on observations of Keller (1989, Brazos River sections), Canudo et al. (1991, Agost and Caravaca sections); El Kef data are chosen from the higher occurrences of Keller (1988) and this study; Millers Ferry data are from this study.

of architectural construction (Olsson et al., 1992). Features such as spine holes, gametogenetic calcification, and calcite plaque buildup indicate a nearly pristine state of preservation. More important, the Millers Ferry sequence contains a sequential biostratigraphy. The stratigraphic occurrences of the evolving Paleocene species are consistent with their phylogenetic relationships, with primitive species occurring earlier than the advanced species (Liu and Olsson, 1992; Olsson et al., 1992). The paleontologist using the graphic correlation technique would chose this section as the initial standard reference section and would then build on it by iterative comparisons to construct the composite reference section.

In order to illustrate the preservational problems with the FO data set of MacLeod and Keller (1991a, b) two of the more important sections they used in graphic correlation, the El Kef section and the DSDP Site 577 section, are compared to the Millers Ferry section. Figure 5 is a

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Table 1 -Species occurrence datums of the early Pa- leocene planktonic foraminifera at Millers Ferry, El Kef and DSDP Site 577. The numbers refer to horizons above the K/P boundary in centimeters.

Mil- lers El DSDP

Taxon Datum Ferry Kef 577

Praemurica taurica FO a 44 82 0 Parvularugoglobigerina eugubina FO b 59 37 0 Globoconusa daubjergensis FO c 59 77 0 Woodringina claytonensis FO d 59 87 0 Eoglobigerina eobulloides FO e 59 87 0 Eoglobigerina edita FO f 59 * 0 Globanomalina planocompressa FO g 123 82 0 Woodringina hornerstownensis FO h 123 102 0 Eoglobigerina trivialis FO i 141 102 0 Chiloguembelina midwayensis FO j 215 82 6 Parasubbotina pseudobulloides FO k 215 * 42

* Eoglobigerina edita, E. eobulloides and Parasubbotina pseudobulloides cannot be differentiated due to preservational compaction of the tests.

conventional stratigraphic correlation which shows the FO of 11 lower Danian planktonic taxa in the three sections. Table 1 gives the datum horizons above the K/P boundary in each section. Figure 5 clearly shows that several correlation lines between the El Kef section and the Millers Ferry section intersect each other and that 9 FO's overlap in the basal Danian at Site 577. Obviously, some of the FO's of Paleocene species at El Kef are delayed and some are in advance compared to their occurrence at Millers Ferry. This can be due to taphonomy, inconsistent tax- onomy or biogeography. A biogeographic effect is unlikely because there was no apparent geographic barrier separating the Mediterranean and the Gulf of Mexico and both localities were in the same climatic belt during the earliest Paleocene. Preservational effect is the major cause. Since the microfossil assemblage at El Kef is poorly preserved due to severe dissolution in Zones P0 and lower Pa, some species, such as Globanomalina archeocompressa, Para- subbotina aff. pseudobulloides and Praemurica taurica which occur in Zone P0 at Millers Ferry, are absent from El Kef. In addition, certain descendant species occur earlier than the ancestral species (e.g., at El Kef, the FO of Chiloguembelina midwayensis is below the FO of Wood- ringina claytonensis which is the ancestor of the former species), suggesting that the ancestral species is not preserved. In addition, the occurrences of most species at El Kef, except for Parvularugoglobigerina eugubina and Guembelitria cretacea, are represented by only one or, at most, a few individuals and exhibit an irregular abundance pattern or a patchy distribution in Zones P0 and lower Pa. The irregular occurrence of some species can also be due to taxonomic inconsistency. Because of compaction, the

tests of Eoglobigerina eobulloides, E. edita and Ps. pseudobulloides are difficult to differentiate.

The 9 overlapping FO's at the base of the Danian at Site 577 (Fig. 5) are generally interpreted in graphic correlation as strong evidence of a hiatus. MacLeod and Keller (1991b) did indeed explain it as a dissolutional hiatus. Their interpretation is based on three facts: Zone Pa overlies directly the Upper Cretaceous; overlapping first occurrences of several Paleocene species; and an abrupt decrease in the carbonate content at the base of Paleocene. We seek an alternative explanation. Since the sedimentation rate during the earliest Danian is 0.116 cm/kyr and the duration of Zone PO is estimated to be 50,000 years (Berggren et al., 1985), the thickness of Zone P0 at DSDP Site 577 would be at most 5.80 cm. Estimation of the magnitude of vertical mixing by bioturbation or winnowing is from 17 to 40 or even more than 90 cm in pelagic sediments (Glass, 1969). Therefore, if Zone P0 was deposited, it would be mixed with and inseparable from Zone Pa or the Upper Creta- ceous. Another possible case for mixing of Zones P0 and Pa is drilling disturbance. D'Hondt and Keller (1991) found that Par. eugubina occurs in the Upper Cretaceous at DSDP Site 577 due to drilling disturbance. In this case, graphic correlation can not resolve whether the absence of a very thin layer is due to mixing or nondeposition.

The taphonomic difficulties outlined above, therefore, compromise the FO data set used by MacLeod and Keller (1991a, b) in graphic correlation. Incorporation of the FO data from Millers Ferry could significantly improve the rigor of this technique. However, the Millers Ferry data should also be compared to other stratigraphic sections with similar preservation to increase the degree of confidence on the first occurrences of taxa. In graphic correlation the range of a taxon is continually adjusted in the composite reference section until additional new information does not alter the range. This procedure is successful in comparing sections where the relative rates of sedimentation are more or less constant. Graphic correlation has been successful where the rates are high enough so that the range of a taxon can be judged with a reasonable degree of confidence. In the comparison of deep sea sections and continental margin sections graphic correlation encounters marked differences in the relative rates of sedimentation. In the case of the K/P boundary sections extremely low sedimentation rates are recorded in the lower Danian at DSDP sites (Zachos et al., 1989) whereas in continental margin sections rates of sedimentation are rep- resentative of normal clastic deposition (Smit, 1982; Kel- ler, 1989; Olsson and Liu, 1990). Condensation or mixing in DSDP sites, especially in the basal Danian, makes range comparison with continental margin sections very uncer- tain.

The FO and LO data set of 6 taxa used by MacLeod and Keller (1991a, b) provides 6 FO's and 4 LO's. Four FO's are very close together so that for practical purposes they represent really 2 data points. Two of the 6 taxa (Parvularugoglobigerina eugubina and P. longiapertura) are regarded as synonomous (Liu and Olsson, 1992; Olsson et al., 1992; among others). Olsson et al. (1992) have point-

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ed out that two other taxa (Eoglobigerina edita and E. pentagona) are inconsistently used by workers (Smit, 1982; Keller, 1988; among others). Consequently, this data set can not be rigorously applied to achieve a Line of Corre- lation as a first step in the construction of a composite reference section.

In summary, the above discussion points out many significant problems in the data sets used by MacLeod and Keller (1991a, b) in applying the graphic correlation technique to identify hiatuses in the lower Danian.

TEMPO AND MODE OF THE K/P MASS EXTINCTION

What is the tempo and mode of the K/P mass extinction? Observations on the extinction/survival of Late Cre- taceous planktonic foraminifera play a major role in in- terpreting this event. Based on observations on the occurrence of Late Cretaceous microfossils in the lower Paleocene, four scenarios have been proposed. Smit (1982) suggested a single catastrophic event caused by an asteroid impact which eliminated all but one planktonic foraminiferal species. Keller (1988, 1989) suggested stepwise extinctions through an interval from the late Maastrichtian to early Paleocene due to multiple causes such as asteroid impact, volcanism, and sea-level fluctuation. Brinkhuis and Zachariasse (1988) concluded a foreshadowed-accelerated and prolonged planktonic foraminiferal extinction which began in the Late Cretaceous while the dinoflagellate flora was basically unchanged through the K/P transition. They advocated sea-level change as the cause of the K/P mass extinction event. From the Braggs section in Alabama, Bryan and Jones (1989) reported that the mass extinction of the macrofauna was earlier than that of the calcareous nannoflora.

What caused these controversies? To answer this ques- tion, one needs to look into the issue with a consistent stratigraphic scheme, and a thorough understanding of the biological and physical factors affecting the fossil record, especially the origin of the Late Cretaceous microfossils occurring in the lower Paleocene.

Based on the observation that Guembelitria cretacea is the exclusive species occurring in the basal Paleocene Zone P0 at El Kef and Gredero, Smit (1982) concluded that a single catastrophic K/P boundary event eliminated all planktonic foraminifera but this species which in turn gave rise to all Paleocene planktonic foraminifera. Smit reported that the occurrence of Upper Cretaceous planktonic foraminifera in the lower Paleocene was restricted to the basal 3 cm. He interpreted them as reworked fractions. However, the occurrence of Cretaceous planktonic foraminifera in many K/P boundary sections is more extensive than what Smit observed. Thus, Canudo et al. (1991) sharply criticized Smit's observation and conclusion.

Keller (1988, 1989) regarded the Cretaceous species in the lower Paleocene at El Kef and Brazos River as indigenous survivor species and proposed a stepwise pattern for the K/P mass extinction event(s). She estimated that approximately one third of the Cretaceous species survived

into the Paleocene. The extinction of 6 Cretaceous species at El Kef was recorded at 25 cm below the K/P boundary. The next 3 succeeding extinction steps were recorded at 5 cm below, at the K/P boundary and 7 cm above the boundary. The other species successively became extinct in Zone Pa (Parvularugoglobigerina eugubina) and in lower Subzone Pla (Parasubbotina pseudobulloides). How- ever, the first four extinction steps coincide with 4 succeeding sample horizons, i.e., Sample #539 (-25 cm), #540 (-5 cm), #541 (0 cm), #542 (7 cm). Consequently, a ques- tion arises: that if the section was more closely sampled, is it possible that more extinction steps would be discov- ered? If stepwise extinctions did indeed occur, the number of extinction "steps" should not be dependent on the number of the samples bracketing the K/P boundary. Appar- ently, the stepwise extinction pattern observed at El Kef by Keller (1988) is sample-biased. Moreover, all species that were suggested by Keller to become extinct before the K/P boundary have been found elsewhere in and/or above the K/P boundary (Liu and Olsson, 1992).

Brinkhuis and Zachariasse draw the K/P boundary at El Kef at 12 cm (sample #543) above the the GSSP K/P boundary (sample #541) and report the first extinctions in Sample #541. Their conclusion can be interpreted as initial extinction at the very end of the Cretaceous followed by gradual extinctions of surviving Cretaceous species. As pointed out, this conclusion is simply the result of arti- factually treating the occurrence of Cretaceous species in the lower Paleocene as indigenous survivor species.

The highest occurrence of Cretaceous species in lower Paleocene strata is observed at Millers Ferry, Alabama, where Cretaceous planktonic foraminiferal species contin- uously occur into Subzone Pla (Parasubbotina pseudobulloides), some 350 cm above the K/P boundary. How- ever, Liu and Olsson (1992) provide a contrasting conclusion based on several independent criteria for recognizing survivor species from the reworked specimens of Cretaceous species. On abundance change, paleoecologic adaptation, phylogenetic relationship to Paleocene species and a synchronous last appearance in the Paleocene, G. cretacea is undoubtedly a survivor species as Smit (1982) and other workers concluded. On phylogenetic criteria, two species of a Hedbergella stock (H. monmouthensis and H. holmdelensis) are also identified as survivor species. All other species are considered reworked fractions from the Upper Cretaceous. We here provide two additional independent criteria, species range and population size variation, to support our previous conclusion.

We contrast the ranges of all Cretaceous planktonic foraminiferal species occurring at different localities (Millers Ferry, El Kef, Brazos River, Gredero, Agost and Site 577) based on published data (Keller, 1988, 1989; Brinkhuis and Zachariasse, 1988; Canudo et al., 1991) and our observations to analyze the occurrence of Cretaceous species in the lower Paleocene. These sites were in the same climatic belt with no apparent geographic barrier separating them during the K/P transition. The paleoenvironments of these sites are different (Fig. 4). If a species survived into the Paleocene, its last occurrence should be approx-

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imately synchronous at these localities. This is, indeed, the case for G. cretacea. On the contrary, the ranges of the other Cretaceous species in six K/P boundary shelf sections at 5 localities are different, being highest for all species at Millers Ferry. For most Cretaceous species, their Paleocene ranges are shortest in the El Kef, Agost and Gredero sections (Fig. 6). All latest Cretaceous species occur but are restricted to the basal few centimeters of the Paleocene at DSDP Site 577. Biogeographically, these highest occurrences at different sites are irregular. How- ever, they are directly related to depositional environment, with the highest occurrences at the shallowest water pa- leoenvironment and lowest occurrences at deep sea sites. It is unlikely that surviving Cretaceous species would become extinct diachronously at these sites. The only logical explanation is that these species are reworked specimens.

We use population ratio change as an additional independent indicator of survivorship. Since the proposed surviving Cretaceous species include small-sized biserial het- erohelicids (Heterohelix and Pseudoguembelina) which are very abundant in the Upper Cretaceous, it is possible to quantitatively approach the reworking/survival issue by using population ratios between Late Cretaceous species populations. The reworking potential of two extinct species that are equal in individual size and preservational potential (susceptibility to dissolution and physical dam- age) should be similar. If one species survived, its population size in the lower Paleocene would be composed of both the reworked and indigenous surviving fractions while that of the extinct species consists only of the reworked fraction. As a result, the relative abundance of the survivor species to that of the extinct species would significantly increase after the extinction of the other species. This is the case for Guembelitria cretacea. Four Late Cretaceous heterohelicid survivor species of Keller (1988, 1989), which are similar in size and have a large population size in the Late Cretaceous, are chosen for this analysis. Among these, Pseudoguembelina costulata and Heterohelix striata were believed to have become extinct during a second phase of K/P extinctions in early Biochron Pa, then H. navarroensis and H. globulosa successively disappear during the gradual extinction period in middle and late Biochron Pa (Keller, 1988, 1989). If the extinctions of these species were indeed at different steps, the relative abundance of the last species to go extinct, H. navarroensis and H. globulosa, would significantly increase after the extinction of P. costulata and H. striata. On the contrary, Table 2 clearly shows that the relative abundance of these Cretaceous species in the Zone P0-Pa interval do not significantly shift away from their Upper Cretaceous values. Only H. globulosa decreased by 10% (from 71.83% to 61.42%) during the interval from 97 cm (sample #338) to 199 cm (sample #324) above the K/P boundary. This variation is statis- tically insignificant and can be explained as due to its more fragile test which decreases its preservational potential compared to the other three species. The population ratio values suggests that none of these four are survivor species.

Our study leads to the conclusion that all specimens of Cretaceous planktonic foraminiferal species, except

Guembelitria cretacea, Hedbergella monmouthensis and H. holmdelensis, are reworked fractions from the Upper Cretaceous rather than survivor species. Consequently, the stepwise extinction of Late Cretaceous planktonic foraminifera as proposed by Keller and her co-workers (Can- udo et al., 1991) as well as foreshadowed and prolonged extinctions of Brinkhuis and Zachariasse (1988) are not substantiated by the data from the Millers Ferry and other K/P boundary sections.

In discussions of causal mechanisms for the K/P boundary event most of the attention has focused on the extinction of Cretaceous species and the possibility of survivor Cretaceous species in the early Danian. We have argued that except for Guembelitria cretacea, Hedbergella holmdelensis, and H. monmouthensis (we recognize that Tubitextularia, a planktonic form in the southern oceans, does range from the Cretaceous into the Paleocene. It is, however, not related to any of the Paleocene taxa discussed in this study) all other oceanic Cretaceous species became extinct at the K/P boundary. The radiation of planktonic foraminifera which began following the K/P boundary event is equally significant in analyzing the geologic history of this event.

It is now clear that important radiations began among planktonic foraminifera in the earliest Danian among guembelitriids, the globorotalids, and the globigerinids (Hemleben et al., 1991; D'Hondt, 1991; Liu and Olsson, 1992; Olsson et al., 1992). Within the guembelitriids the Cretaceous survivor species, Guembelitria cretacea, expanded in abundance along continental margins and extended into the open ocean environment, not previously occupied by it in the Maastrichtian. Woodringina, Chilo- guembelina, Globoconusa, and Parvularugoglobigerina (microperforate planktonic foraminifera) were the first to radiate. The initial part of this radiation was rapidly followed by a radiation of the cancellate-walled spinose globigerinids (Eoglobigerina, Parasubbotina Olsson et al., 1992, and Subbotina). This radiation introduced an en- tirely new adaptation among planktonic foraminifera; that is the first use of spines (Hemleben et al., 1991), an adaptation that enhanced the ability of this group of foraminifera to capture actively moving organisms in the water column. Two other groups also began independent radiations concurrently: the smooth-walled globanomalinids and the cancellate-walled nonspinose praemuricates. When the K/P boundary is viewed in this context, it is clear that an event of very large magnitude had occurred. These radiations are derived from the three Cretaceous species noted above. Thus, the phylogeny and radiation of all other Cretaceous planktonic foraminifera had ended with the K/P event, irregardless of the possibility of any survivors. A radiation of the breadth exhibited by the early Danian planktonic foraminiferal species would appear to indicate an environment that had been radically altered and/or vacated by extinctions due to an event that caused severe environmental stress. The data that show the sharp re- duction in abundance of Cretaceous species at the K/P boundary suggests a complete collapse of the ecosystem. The concomitant early Danian radiations would appear to

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CRETACEOUS-PALEOGENE BOUNDARY AND PLANKTONIC FORAMINIFERA 137

Table 2-Relative abundance (> 63 ,m fraction) of 4 Late Cretaceous heterohelicid planktonic foraminiferal species across the K/P boundary at Millers Ferry, Alabama. These species are: Heterohelix striata, H. navarroensis, H. globulosa and Pseudoguembelina costulata.

H. H. P. Cm H. H. P. globu- navar- H. costu-

Sample above globu- navar- H. costu- Total losa roensis striata lata # K/P losa roensis striata lata counts % % % %

Basal Paleocene 324 199 78 16 17 16 127 61.42 12.60 13.39 12.60 328 173 75 13 15 14 117 64.10 11.11 12.82 11.97 334 123 91 14 13 15 133 68.42 10.52 9.77 11.28 338 97 102 15 10 15 142 71.83 10.56 7.04 10.56 340 81 99 14 11 12 136 72.79 10.29 8.09 8.82 342 59 124 20 12 15 171 72.51 11.70 7.02 8.77 344 44 130 22 14 14 180 72.22 12.22 7.78 7.78 346 30 96 19 11 12 138 69.57 13.77 7.97 8.70 348 16 78 16 9 11 114 68.42 14.04 7.89 9.65 350 0 110 20 13 15 158 69.62 12.66 8.23 9.49 K/P .. - -

U. Cretaceous 352 -18 196 35 27 28 286 68.53 12.24 9.44 9.79 354 -30 158 32 20 23 233 67.81 13.73 8.58 9.87 356 -46 112 24 14 19 169 66.27 14.20 8.28 11.24 362 -74 134 22 17 22 195 68.72 11.28 8.72 11.28 366 -112 126 26 16 20 188 67.02 13.83 8.51 10.64

indicate an adaptation to a new ecosystem quite different from the old. Thus, if the entire structure of the oceanic surface waters sharply changed due to the K/P boundary event extreme environmental pressures would have caused massive extinctions among planktonic foraminifera with very few survivors. Generalized species such as those of Hedbergella or opportunistic species such as Guembelitria cretacea (Liu and Olsson, 1992) would be best adapted to survive. Multiple levels of extinctions, whether caused by multiple events or not, would not fit this scenario nor does the pattern of radiation of the early Danian planktonic foraminifera. Taken in total, there is compelling evidence from the K/P stratigraphic record that mass extinction of Cretaceous planktonic foraminifera occurred during the boundary event.

DISCUSSION

In deep sea sites the K/P transition is marked by an abrupt decline in sedimentation rate whereas in outer shelf and upper slope sections the K/P boundary is marked by a lithological change from carbonate dominated lithofacies to black clay sediments accompanied by an abrupt decrease in sedimentation rate. In middle and inner shelf environments the K/P transition is represented by a possible disconformity. The K/P boundary layer in all paleoenvironments is characterized by low carbonate content. In the pelagic or semipelagic environments this can only be related to the mass extinction of calcareous plankton. In the continental shelf sections a lithologic change

is apparently linked to both a decrease in carbonate production and an increase in terrigenous input related to sea level.

In the Gulf Coast, Hansen et al. (1987) proposed that the sand bed lying on the Corsicana Formation at Brazos River, Texas is a tsunami deposit which resulted from an asteroid impact in the Gulf of Mexico. Attention has been directed at possible impact sites in Haiti in the Caribbean (Florentin et al., 1991; Sigurdsson et al., 1991) and in Yu- catan, Gulf of Mexico (Smit et al., 1992). A K/P boundary "monolith bed" composed of coarse clastics with indicators of strong currents and located in a pelagic sequence at Arroyo el Mimbral in Yucatan is interpreted by Smit et al. as caused by an impact generated abrupt wave disturbance which washed back debris from the North American continent into an undisturbed deep Gulf basin. They believe that the monolith bed is incompatible with the hy- pothesis of a K/P boundary eustatic fall.

Nevertheless, the sedimentological and micropaleonto- logical data across the K/P boundary in Texas and Ala- bama discussed previously indicates that sea level was at lowstand during the K/P transition. The "tsunami bed" at Brazos has been shown to be Paleocene age and apparently equivalent to the Clayton Basal Sands in Alabama and the Littig Member in Texas. A sea-level fall at the K/P boundary, therefore, can not be ruled out but neither can a tsunami event. Since sea level had already started to fall during the late Maastrichtian, sea level was low both before and after the boundary event. The undulating surface that marks the top of the Cretaceous in the Gulf



OLSSON & LIU

Coastal Plain (Fig. 4) is possibly the result of a tsunami event since the hiatus associated with it, if present, appears to be very short in duration. The collapse of carbonate production due to mass extinction at the boundary would have provided the sedimentary regime along with falling sea level to allow the lowstand sands to accumulate over the Cretaceous formations, perhaps assisted initially by a tsunami wave. Shearing of the top of the Cretaceous formations by a tsunami wave in the Gulf Coastal Plain may have sorted, transported, and deposited sand along the Cretaceous shoreline where it could have been redeposited in the earliest Danian. Backwashing of some of these sediments by a tsunami wave into the deep Gulf basin is a distinct possibility.

An eustatic sea-level fall at the K/P boundary, if suffi- ciently large in magnitude, would cause a similar stratigraphic scenario with nearshore sands being deposited over an eroded shelf as sea level starts to rise again. During sea level fall turbidites would be deposited in the deep Gulf basin. Sea level must have fallen and again risen very rapidly if this is, indeed, the scenario. Thus, the magnitude of a sea-level fall and the subsurface distribution of shelf margin and fan deposits are critical to supporting this scenario as well as a plausible mechanism for a rapid drop in sea level and the simultaneous mass extinction of oceanic calcareous plankton. The stratigraphic data of boundary sections suggests that only shallow shelf environments were affected by an erosional event. A tsunami event cer- tainly would have a maximum effect in shallow shelf depths.

In conclusion, examination of the Late Cretaceous species occurring in six key K/P boundary sections indicates that all Late Cretaceous planktonic foraminifera, except three species which are phylogenetically related to the evolution of all Paleocene planktonic foraminifera, became extinct at the very end of the Cretaceous. This mass extinction is a single catastrophic event which eliminated approximately 94% (~50 of 53) of the Late Cretaceous planktonic foraminiferal species.

The neritoplanktonic species, Guembelitria cretacea, is the only species that significantly increased its abundance and, in turn, gave rise to all microperforate planktonic foraminifera in the Paleocene. The other two survivor species, Hedbergella monmouthensis and H. holmdelensis, are very rare in the lowermost Paleocene as observed in the Millers Ferry sequence. Perhaps, these two related species were also severely stressed during the mass extinction. In Biochron P0, they gave rise to three major groups of early Paleogene planktonic foraminifera: cancellate walled nonspinose, cancellate walled spinose and smooth walled planktonic foraminifera. The previously proposed extinction patterns, either stepwise or foreshadowed and prolonged, are combined artifacts of equivocal placement of the K/P boundary at different sections and treatment of reworked Cretaceous species in the lower Paleocene as indigenous survivor species. This conclusion does not agree with Smit (1982) on the number of Late Cretaceous survivor species but it is consistent with his conclusion on the tempo and mode of the K/P mass extinction of planktonic foraminifera.

ACKNOWLEDGMENTS

We thank W. A. Berggren for his review and helpful suggestions on improving this paper. This study has also benefitted from the suggestions and critical comments from two anonymous reviewers. Dr. D. Habib kindly supplied samples from the basal Paleocene at Braggs and Mussel Creek in Alabama for biostratigraphic study. We thank ODP on-shore staffs for providing the DSDP and ODP samples for this study. Millers Ferry Core samples were provided by the U.S. Corps of Army Engineers, Alabama. Studies by the senior author were supported by the In- stitute and Museum for Geology and Paleontology, Uni- versity of Tubingen, Germany while he was on sabbatical during 1990-91. The Department of Geological Sciences, Rutgers University, Rutgers Research Council and Rutgers Graduate Student Research Grant provided support for C. Liu.

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ACCEPTED AUGUST 25, 1992

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