marine cretaceous-paleogene biofacies and ichnofacies in southern tibet, china, and their...

ELSEVIER Marine Micropaleontology 32 (1997) 3-29

Marine Cretaceous-Paleogene biofacies and ichnofacies in southern Tibet, China, and their sedimentary significance

Zhou Zhicheng a,*, Helmut Willems b, Zhang Binggao a

a Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing 210008, China ’ Fachbereich 5-Geowissenschaften, Universitiit Bremen, 28334 Bremen, Germany

Received 1 June 1996; accepted 1 December 1996

Abstract

The study of biofacies is of great significance not only for biostratigraphy, but also for the interpretation of sedimentary environments and the reconstruction of paleogeography. The areas of Tuna, Gamba and Tingri of southern Tibet, China, were located in the north margin of the Indian Plate in the Cretaceous and Paleogene. The marine Cretaceous and Paleogene biofacies there can be divided into four groups using the following criteria: (a) the benthic biota and biogenic structures in the sediments; (b) the paleoecology of the main fossils; (c) the genesis of fossils. The four groups are euxinic biofacies, nektonic and planktic biofacies, benthic biofacies and redeposited biofacies, which include 24 main biofacies types. Like biofacies. ichnofacies are important indicators of paleoenvironments. The Cretaceous-Paleogene trace fossils in the Tuna and Gamba areas are attributable to Nereites, Zoophycos, Cruziana and Skolithos ichnofacies. Nereites ichnofacies indicates an open marine basin environment. Zoophycos and Cruziana ichnofacies indicate shelf margin and open sea shelf environments. The Skolithos ichnofacies reflects the environments of sand barrier deposited in the intertidal zone and the upper part of subtidal zone. The Zoophycos ichnofacies can further be subdivided into Thalassinoides-Planolites and Zoophycos-Chondrites subichnofacies which reflect aerobic and disaerobic conditions, respectively. The development of biofacies and ichnofacies in vertical profiles shows the sedimentary history from the Early Cretaceous to the Paleogene in Tuna, Gamba and Tingri areas. A regression occurred in Tuna and Gamba from the Neocomian to the Early Paleocene. The sedimentary environments went from euxinic basin and open marine basin, shelf margin, open sea margin, platform slope, platform margin, open marine platform to the sand barriers deposited in the intertidal zone or the upper part of subtidal zone. A transgression took place at the beginning of the middle Paleocene. The carbonate deposits formed in the open marine platform and open sea shelf environments overlay the sandstones of the Jidula Formation. The termination of marine deposits in these areas began in the Thanetian. The sedimentary history in the Tingri area differed from that in the Tuna and Gamba areas especially during middle Maastrichtian to Early Paleocene times. The sedimentary process in Tingri during this period took place in the lower part, the middle part of a fan and seaward prograding delta plains. The marine deposition in the Tingri area ended in Lutetian later than in the Tuna and Gamba areas.

Keywords: Cretaceous; Paleogene; biofacies; ichnofacies; Southern Tibet, China

* Corresponding author. E-mail: [email protected]

0377-8398/97/$17.00 I@ 1997 Elsevier Science B.V. All rights reserved. PIZ SO377-8398(97)0001 1-X

4 Zhou Zhicheng et al. /Marine Micropaleontology 32 (1997) 3-29

1. Introduction

The Tuna, Gamba and Tingri areas of southern Ti- bet, China were located at the northern margin of the Indian Plate and belonged to the southern subzone of the Tethys-Himalaya during the Cretaceous. Many geologists have been attracted there and a great deal of researches on palaeontology, stratigraphy, sedi- mentology and tectonics has taken place over the last three decades because of the important tectonic location and the developed marine Cretaceous and Paleogene strata (He Yan et al., 1976; Wang Yujing, 1976; Wen Shixuan, 1974; Wan Xiaoqiao, 1987; Xu Yulin et al., 1989; Yin Jixiang, 1988; Liu and Ein- sele, 1994). The authors have made many scientific expeditions to southern Tibet since 1966 and they have obtained a large quantity of field samples; the results on biostratigraphy, carbonate microfacies and ichnology have been published (Zhou Zhicheng and Zhang Binggao, 1992; Willems and Zhang Binggao, 1993a,b; Willems, 1993; Zhang Binggao et al., 1994; Zhou Zhicheng, 1997). This paper is based on the former research work.

The Cretaceous to Paleogene is an important geo- historic time period. The Indian Plate drifted rapidly northwards and collided with the Eurasian Plate. During this event, great changes in paleogeogra-

phy and sedimentary environments took place and gave rise to the development of biofacies and ichnofacies. This paper describes the main types of Cretaceous and Paleogene marine biofacies and ichnofacies in detail and suggests their environmental significance. Also on the basis of environmental analysis of the biofacies and ichnofacies, the marine sedimentary history in these areas during the Cretaceous-Paleogene is reconstructed.

2. Materials and methods

The samples for the studies of biofacies come from sections At and Gulu of the Tuna area, sections A, K, D, F, Zp and Zm of the Gamba area and sections L, M, Entong and Longjiang of the Tingri area. The samples and field photos for the studies of ichnofacies mainly come from section At of Tuna and sections A, K, D of Gamba (Fig. 1).

The following criteria for the classification of biofacies have been taken into consideration: (a) the existence of benthic biota and biogenic structures in sediments (the biofacies is termed euxinic biofacies if there are no benthic fossils and biogenic structures in the sediments); (b) the paleoecologic features of the main fossils (according to this criterion, two groups of biofacies, nektonic and planktic biofacies

1 1 T-T /B E ‘I’ b I

-6amba4 3 - pT,HIMALAYA MTS.

0 30 60 km 1 Y adong

‘i. dj-

Fig. 1. Map showing the localities of sections. I = Section At; 2 = Section Gulu; 3 = Section A; 4 = Section K; 5 = Section Section F, 7 = Section Zp; 8 = Section Zm; 9 = Section L; 10 = Section M; II = Entong Section; 12 = Longjiang Section.

D: 6 =

Zhou Zhicheng et al. /Marine Micropaleontology 32 (1997) 3-29 5

and benthic biofacies are classified); and (c) the genesis of fossils (the fossils which were derived from resedimentation are referred to redeposited biofa- ties).

The classification of ichnofacies described in this paper is based on the classification by Seilacher (1967) Frey and Seilacher (1980) and Ekdale et al. (1984). The Cretaceous and Paleogene trace fossils can be divided into Nereites, Zoophycos, Cruziana and Skolithos ichnofacies.

When the biofacies and ichnofacies are used to interpret the sedimentary environments, the other factors such as diagenetic features and the textures and structures of rocks are also considered.

3. Stratigraphy

According to Willems and Zhang Binggao (1993a), the marine Cretaceous-Paleogene sequences in Gamba and Tuna are formed of the Gamba Group (Neocomian-Santonian), the Zongshan For- mation (Campaniart-Maastrichtian), the Jidula For- mation (Maastrichtiar-Paleocene), the Zongpu For- mation (Paleocene-Thanetian) and the Zongpubei Formation (Thanetian).

The Gamba Group is composed of three formations (in ascending order): Formation 1, the Gan- badongshan Formation, at least 760 m thick, consisting of siltstones and claystones intercalated by turbiditic sandstones; Formation 2, the Chaquiela Formation, consisting of about 560 m siltstones and claystones interstratified by many sandstone turbidites and sandstone dykes, and with increasing intercalations of lilmestones and marlstones in the upper part; Formation 3, the Ganbacunkou Forma- tion consisting of an interstratification of pelagic limestones, marlstones and fewer claystones. The Zongshan Formation with a thickness of at least 220 m is formed by three (in Gamba) or four (in Tuna) limestone packages rich in fossils, separated by calcareous marlstones and marlstones. The 150-170 m thick Jidula Formation is composed of a sequence of ferruginous sandstones, in the middle part locally interbedded by black limestones and claystones. The Zongpu Formation., which is at least 250 m thick, comprises a variety of well-bedded limestones, but mainly nodular limestones and marlstones. The non- marine Zongpubei Formation with a thickness of at

least 160 m (in Gamba) consists of greenish-grey marlstones and red clay and siltstones.

Comparing the areas of Gamba and Tingri, some differences in the lithostratigraphic sequence can be recognized (Willems and Zhang Binggao, 1993b). In Tingri, the marine Cretaceous and Paleogene include the Gamba Group (Vraconian-Santonian), the Zongshan Formation (Santonian-Maastrichtian), the Zhepure Shanpo Formation (Maastrichtian- Paleocene), the Jidula Formation (Paleocene) and the Zhepure Shan Formation (Paleocene-Eocene). The Gamba Group with an investigated thickness of at least 625 m consists of marlstones which are intercalated by continuously increasing layers of calcareous marlstones and limestones at the top. In compari- son to the type locality at Gamba, the Zongshan Formation is cut off by an erosional unconformity. The following 225 m thick Zhepure Shanpo For- mation in its lower part is composed of marlstones interbedded by turbiditic sandstones (often with ‘fin- ing upward’ cycles), and in the upper part it consists of mixed carbonatic siliciclastic resediments. The Jidula Formation consists of about 100 m of calcareous sandstones intercalated by nodular marly sandstones and glauconitic sandstones. The 440 m thick Zhepure Shan Formation forming the summit of the Zhepure Mountain Range can be subdivided into five members (in ascending order): Dolomitic limestones and dolor&es (Member I); thick-bedded limestones (members II-III); nodular limestones and calcareous marlstones (Member IV); and very mas- sive limestones (Member V).

4. Biofacies

The 24 biofacies described in this paper do not cover all biofacies types, but are representative of the main biofacies types in the Cretaceous-Paleogene strata in the Tuna, Gamba and Tingri areas of southern Tibet. The environmental interpretation of the biofa- ties can provide the most important information about the sedimentary features and the paleogeography.

4.1. Euxinic biofacies

4.1.1. BF-I euxinic biofacies The euxinic biofacies coincides with the Letal-

pantostrat biofacies of Schafer (1963). There are no


benthic organisms or biogenic sedimentary structures in the continuous sedimentary sequence. This biofacies reflects a kind of stagnant, anaerobic and reducing condition which was not a suitable habi- tat for the benthic organisms. The black shale and mudstone in the Formation 1 of the Gamba Group contains this biofacies.

the limestones indicates depositional conditions with good water circulation and oxygenation.

4.2.2. BF-3 calcispheres (Plate I, 2)

4.2. Nektonic and planktic biofacies

4.2.1. BF-2 planktic foraminifers (Plate I, 1)

The BF-3 is characterized by predominant calcispheres and coexisting calpionellids. The former consist mainly of Pithonella sphaerica (Kaufmann) and P. ovalis (Kaufmann) (Plate I, 2). The latter Calpionellidae Bonet, includes Calpionella Lorenz, Stenosemellopsis Colom, Tintinnopsella Colom, En- dothicollaria Zhang et al. (Plate I, 3).

The fauna is mainly composed of planktic foraminifers (Heterohelix sp., Globotruncana, Dicarinella, Rotalipora, Hedbergella, Whiteinella, etc). The other components include a small quantity of smaller benthic foraminifers (Textularia), calcispheres, calpionellids and some fossil fragments such as echinoderms, molluscs, inoceramids and ostracodes derived from shallow water environments. The fossils occur in strongly bioturbated wackestones.

The other fossils consist of a small number of planktic and smaller benthic foraminifers and some fossil fragments which came from shallow water environments.

This biofacies is indicative of the environments with a good water circulation including the shelf margin, open sea shelf and platform slope.

4.2.3. BF-4$laments (Plate I, 4) This biofacies occurs in the environments of shelf Filaments originate from juvenile planktic or

margin, open sea shelf (Tuna and Gamba) and open pseudoplanktic larval stages of pelecypods (Fhigel, marine basin (Tingri) which are under the normal 1982). Wilson (1975) interpreted the filamentous wave base (Figs. 2-4). The strong bioturbation in micrite as ‘offshore basinal environment’. The bio-

FZ FACIES ZONES AND SUB -ZONES

: CUT - OFF LAGOONS, C

7 B RESTRICTED LAGOONS

A QUARTZ SAND BARRIERS

NOUSLY INFLUE PLATFORM PLATFORM

;.,

$px$~~ 4

* . . . . . . . . _r;. .:.. . PLATFORM SLOPE / PROXIMAL CONTINENTAL SLOPE

3 OPEN SEA SHELF (::~t~::::::::::..::[ffl 2 . . . . . . . . . . . . . . . 1

SHELF MARGIN / DISTAL CONTINENTAL SLOPE

Fig. 2. Summary of the eight facies zones (FZ) and further sub-zones differentiated in the facies models of Figs. 3 and 4 (after Willems, 1993).


NWBI SWB

M .4

2 zcwphyc~

I I I I H

Fig. 3. Schematic environmental interpretation of biofacies types and ichnofacies of the Tiina and Gamba areas. NWB = normal wave base; SWB = storm wave base.

NWB sWB

Fig. 4. Schematic environmental interpretation of biofacies types of the Tingri area. NWB = I normal wave base: SWB = storm wave base.

facies in Gamba occurs in the sandy limestone of the Chaquiela Formation in which trace fossils of Cruziana ichnofacies have also been found. The co-

existing fossils are mainly echinoderm fragments. The BF-4 limestone was deposited in the shelf margin or open sea shelf environments.


The filamental biofacies in Tingri has been found in the marls at the upper part of the Gamba Group. The biota is briefly comprised of filaments, calcispheres and planktic foraminifers, and the BF-4 is intercalated in BF-2 in vertical section. The biofa- ties indicates an open sea basin or open sea shelf environments.

4.2.4. BF-5 ammonites Ammonites are important fossils in Cretaceous

biostratigraphy. They are mostly found in the detrital rocks and marls of the Gamba Group. The mi- nor biota include bivalves, gastropods, echinoderms, belemnites and planktic foraminifers.

Trace fossils of Nereites and Cruziana ichnofacies have also been found in the deposits which often contain the ammoritic biofacies. The biofacies and the coexisting ichnofacies reflect the depositional environments of open marine basin, shelf margin and open sea shelf.

4.3. Benthic biofacies

4.3.1. BF-6 Goupillaudina-Sulcoperculina (Plate ZZ, 1)

The biota are characterized by the simultaneous occurrence of benthic organisms and planktic organisms. The former include benthic foraminifers Goupillaudina, Sulcoperculina (Plate II, 1) and Ne- ojlabellina and other fossils such as molluscs, ostracodes, brachiopods and echinoderms. The latter are composed of planktic foraminifers (Globotruncana), calcispheres, calpionellids and filaments. The fossils are found in strongly bioturbated wackestone, packstone and floatstone.

The composition of the biota shows that the biofa- ties is the typical transitional type of the benthic and planktic biofacies. The depositional environment was situated on the platform slope between the normal wave base and the storm wave base.

4.3.2. BF-7 Orbitoides-Sulcoperculina (Plate ZZ, 2) The benthic foraminifers include Orbitoides, SuZ-

coperculina, Goupillaudina, Miliolidae, Textularia and Rotaliidae. The other biota include echinoderms, molluscs, brachiopods, ostracodes and bryozoans. Some planktic foraminifers and calcispheres are also present. The bioclasts with micrite envelopes occur in the bioturbated wackestone, grainstone and packstone. The limestone also contains some fine-grained well rounded quartz.

BF-7 indicates shallower environments on the platform slope and high-energy shoals located at the platform margin.

4.3.3. BF-8 Orbitoides-MiZioZidae The biofacies is characterized by high diversity

of benthic biota including foraminifers (Orbitoides, Miliolidae, Sulcoperculina, Goupillaudina, Textu- Zaria), echinoderms, brachiopods and Rhodophyceae (Archaeolithothamnium). A few planktic foraminifers and calcispheres are also found in the limestone.

The features of biota and diagenic alternation indicate that sedimentation took place in the high- energy shoals above the normal wave base which was located at the platform margin.

4.3.4. BF-9 Orbitoides-Omphalocyclus (Plate ZZ, 3) The predominant components of the biota are

the larger benthic foraminifers Orbitoides media (d’Archiac) and Omphalocyclus macroporus (La- marck). The minor foraminifers include Sulcoper- culina, Rotaliidae and Miliolidae. Other biota include echinoderms, brachiopods, molluscs and calcareous algae (MarineZZa Zugeoni, Corallinaceae).

The bioclasts in grainstone, packstone, floatstone and rudstone were influenced by strong micritization. Sometimes, there was a high input of terrestrial materials during sedimentation. The sedimentation occurred in the high-energy shoals on the platform margin or a terrestrially influenced platform.

Plate I 1. Planktic foraminifers (Globotruncana) with some calcispheres. BF-2. Section D. Lower Zongshan Fm., Campanian. x 20. 2. Calcispheres (Pithonellu sphaerica, II ovalis), BF-3. Section At. Sample At38, Campanian. x6.5. 3. Calpionellids, BF-3. Section At, Sample At36, Campanian. x200. 4. Filaments with a few echinoderm fragments. BF-4. Section A, Fm. 2 of Gamba group. x5. 5. Keramsphaeru and Miliolidae. BF-21. Gulu Section, Paleocene. x 10. 6. Miscellanea, Sphaerogypsina and Verneuilina, BF-16. Section M, Sample Mr66, Paleocene. x20.

IO Zhou Zhicheng et al. /Marine Micropaleontology 32 (1997) 3-29


4.3.5. BF-10 rudists (Plate III, 1) The biota is composed of unrounded and un-

sorted rudist fragments (Plate III, 1). The accessory fossils are benthic foraminifers (Orbitoides, Om- phalocyclus, Sulcoperculina and Miliolidae, etc.), echinoderms, brachiopods, Rhodophyceae (Archaeo- lithothamnium) and corals.

The boring structures showing geopetal fabrics can be found in the rudist fragments. Most of the bioclasts have the micrite envelopes. The diagenetic features are characterized by extensive mouldic porosity caused by selective solution.

The typical biofacies was formed in the high- energy shoals at the platform margin where rudist biostromes occasionally developed.

4.3.6. BF-11 Omphalocyclus-calcareous algae The benthic foraminifers consist mainly of Om-

phalocyclus and minor Orbitoides, Miliolidae and Rotaliidae. The calcareous algae include Coralli- naceae, Dasycladaceae and Udoteaceae (Halimeda and Ovulites). Other fossils include echinoderms, molluscs, rudists and sponges.

According to Elf-Aquitaine (1977, p. 24), Udotea- ceans tend to settle in shallow subtidal zones of the inner platform. The high diversity of biota and the presence of Udoteaceans show that this biofacies was produced in the open marine platform environment with good water circulation.

4.3.7. BF-12 Corallinaceans (Plate II, 4) Corallinaceans (Archaeolithothamnium, Litho-

thamnium and Lithophyllum) encrusted other bioclasts such as rudists, corals and sponges, etc. to form rhodoids which show different growth forms: ramose, columnar, globular and laminated.

The accessory biota include other algae (Hali- meda, Ovulites, Solenoporaceae and Dasycladaceae), foraminifers (Omphalocyclus, Orbitoides, Milioli- dae and Rotalidae), echinoderms, ostracodes, inoceramids, rudists, corals, sponges, stromatopoids,

serpulids, bryozoans and gastropods. The b&lasts with micrite envelopes occur in floatstone, rudstone, packstone and wackestone.

According to Bosence (1983, p. 226), the recent rhodoids of Florida occur in the sea grass bank near the coast and adjacent to ‘mud mounds’. The environment of rhodolite in Gamba is similar to that of recent rhodoids. The sedimentation of BF-12 took place on a nearshore marine platform which was influenced by tenigenous elastic input.

4.3.8. BF-13 Ovulites-ostracodes The biofacies is characterized by low biota di-

versity which consist of Ovulites, dasycladaceans, rudists, ostracodes and small-sized foraminifers (Verneuilina, Miliolidae, etc.). The limestones are intercalated in the sandstones of the Jidula Forma- tion and contain quite a few fine-grained to coarse- grained quartz and some pyrite. The contact between these limestones and the underlying Lower Sandstone is marked by an erosional unconformity (Willems and Zhang Binggao, 1993a, fig. 6b).

According to Wray (1977, pp. 136, 155), Ovulites is an inhabitant of shallow subtidal (nearshore) lagoonal environments with temporary terrigenous influx, especially on the downwind side of protecting bars. This situation appears to hold true in the environmental interpretation of BF- 13. The occurrence of limestones, the low biota diversity, the biotic as- pect and the presence of pyrite indicate a typical nearshore lagoonal environment influenced by terrigenous quartz input.

4.3.9. BF-14 Lockhartia-Dasycladaceae (Plate III, 2)

This biofacies is characterized by the abundance and high diversity of biota especially calcareous algae and benthic foraminifers. The former include Dasycladaceae (Neomeris, Trinocladus, Cymopo- ha, Fucoporella, Indopolia, etc.), Acetabulariaceae (Acicularia, Clypeina), Corallineae (Jania, Amphi-

Plate II 1. Goupillaudina and Sulcoperculina with some calcispheres and planktic foraminifers, BF-6. Section At, Sample At47, Campanian. x22. 2. Orbitoides, Sulcopenulina and Goupillaudina, BF-7, Section At, Sample At68, Campanian. x20. 3. Orbitoides and Omphalocyclus, BF-9. Section At, Sample At105, Campanian. x20. 4. The encrusting Archoeolithothamnium shows the globular growth type. BF-12. Section D, Sample Dr49, Maastrichtian. x20.


roa), Udoteaceae (Ovulites, Hulimedu) and the latter include Lockhartia, Miliolidae, Vemeuilinidae, Discocyclina and Sphaerogypsina. Other fossils include echinoderms, bivalves, gastropods, ostracodes, sponges and corals.

The high biota diversity shows that the primary environment had a good water circulation and the sedimentation took place in an open marine platform environment.

4.3.10. BF-15 Lockhartia-Verneuilinidue (Plate III,

3) The biota consist mainly of benthic foraminifers

including Lockhartia, Vemeuilinidae Miliolidae and Textularia, etc. The other biota are echinoderms, ostracodes and calcareous algae (Lithophyllum, Jania and Corallina subtilis). The limestones have been influenced by dolomitization.

The BF-15 is intercalated in BF-14 (Fig. 6). The biota diversity of BF-15 is much lower than that of BF-14. The marine platform was influenced by a small episodic transgression and regression in middle Paleocene. The BF-14 was formed in the trans- gressive stage and the BF-1.5 was formed in the regressive stage. The depositional environment of BF-15 was shallower than that of BF-14.

4.3.11. BF-16 Miscellanea-Ranikothalia (Plate I, 6; Plate III, 5, 6)

This biofacies is characterized by abundant larger foraminifers including Miscellanea, Ranikothalia, Discocyclina, Operculina, Sphaerogypsina and Alve- olina. The small-si.sed foraminifers include Lockhar- tia, Miliolidae and Vemeuilinidae. The other biota are echinoderms, bivalves, gastropods, ostracodes and calcareous algae (Dasycladaceae indet., Furco- porella, Dissocladella, Distichoplax biserials, Litho- porella and Trinocladus).

The described assemblage of larger foraminifera is typical for open marine shelf areas (Hottinger,

1973, p. 445). After Reiss and Hottinger (1984, p. 282), during the middle to Late Paleocene, especially, Ranikothalia associated with Operculina was a characteristic fauna for low-energy and soft bottom environments for water depths from 60 to 90 m. The existence of green algae indicates a good aeration and light penetration. As mentioned above, BF-16 is indicative of open marine shelf or carbonate platform environments.

4.3.12. BF-17 Alveolina-Orbitolites (Plate ZV 2) The biofacies consists mainly of larger fora-

minifers, especially Alveolina and Orbitolites. The other foraminifers include Opertorbitolites, Rotali- idae, Miliolidae and minor Vemeuilinidae and planktic foraminifers. The accessory biota are calcareous algae (Corallineae, Melobesieae indet., Dasy- cladaceae indet., Ovulites and Halimeda, and the fossils are found mainly in wackestone, floatstone and packstone.

According to Reiss and Hottinger (1984, p. 283) and Hottinger (1973, p. 446), the occurrence of alveolinids and Orbitolites indicates a shallow and mod- erately agitated environment. Sedimentation took place in open marine platform or high-energy shoals.

4.3.13. BF-18 Halimeda-echinoderms (Plate IV 1) The biofacies mainly consists of Halimeda and

echinoderms. The foraminifers comprise the benthic small-sized foraminifers (Miliolidae, Vemeuilin- idae, Textularidae and Rotaliidae) and a few planktic foraminifers. Calcareous algae include Dasycladaceae (Pianella succincta), Melobesieae (Palaeolithothamnium) and Udoteceae (Hulimeda, Ovulites). Other fossils include bivalves, gastropods, ostracodes, bryozoans and brachiopods.

The bioclasts with micrite envelopes are found in wackestone, packstone, floatstone and rudstone. The limestones bearing BF-18 interfinger with ooid grainstone. The diagenetic features and the high

Plate III 1. Rudist fragments mainly consisting of radiolitids and hippuritids, BF-10. Section D, Sample Dr31, Maastrichtian. x.5 2. Lockhartia and Dasycladaceae, BF-14. Section Zm, Sample Zm14, Paleocene. x20. 3. Biota mainly consisting of Lockhartia, Verneuilina and miliolids, BF-15. Section Zp, Sample Zpl 1, Paleocene. x20. 4. Assilina. BF-22. Longjiang Section, Eocene. x 10. 5. Miscdanea and Alveolina. BF-16. Section F, Sample Ff33, Paleocene. x20. 6. Miscellanea and Discocyclina, BF-16, Section F, Sample Fr9, Paleocene. x 10.


biota diversity indicate that the sedimentation took place in the shallow and well circulated environment of the open marine platform adjacent to the ooid bars.

4.3.14. BF-19 Pseudolithothamnium-corals The biofacies is characterized by laminar rhodoids

which were formed by the encrusting Pseudoli- thothamnium album and minor Lithophyllum, Litho- thamnium and Archaeolithothamnium. Most rhodoid nuclei are composed of autochthonous or slightly re- worked corals (Pocilloporidae, Poritidae: Goniopora, Alveopora), sponges, Solenomeris and lithoclasts.

The rhodoids are embedded in bioclastic wacke/packstone. Other biota include Corallineae (Amphiroa, Jania), udoteacean (Halimeda), dasycladacean algae (Neomeris, Cymopolia, Clypeina), rotaliids, miliolids, verneuilinids, ostracodes, bivalves, gastropods and echinoderms.

Solution in the freshwater vadose zone led to the intensive vuggy and mouldic porosity. The voids were filled with the granular calcite and vadose silts in a geopetal fabric. The diagenetic features indicate that the sedimentation took place in shallow and restricted environments. Compared with the model of tropic rhodolite environments (Bosence, 1983, pp. 226, 232), the most appropriate sedimentary environment would be shallow subtidal rhodolite bars (patch reefs: FZ6B, Fig. 4) forming a barrier between the open marine platform (FZ6A) and restricted lagoon (FZ7B).

4.3.15. BF-20 Miliolidae-Dasycladaceae (Plate IV 3, 4)

The small-sized benthic foraminifers are composed of miliolids, vemeuilinids and Textularia. Dasycladacean algae occupy the main components of calcareous algae. They are Cymopo- ha, Dissocladella, Furcoporella, Neomeris, Trin- ocladus and Zndopolia (Plate IV, 4). Other algae include Udoteaceae (Halimeda, Ovulites), Melobe-

sieae (Archaeolithothamnium) and Squamariacaea (Pseudolithothamnium album). Other minor biota consist of echinoderms, bivalves, gastropods, serpulids and corals. The b&lasts are embedded in wackestone, packstone and floatstone.

It is suggested that the sedimentary environment was a restricted lagoonal area (FZ7B, Fig. 4) protected from the open marine platform by rhodolite bars.

4.3.16. BF-21 Lockhartia-Keramosphaera (Plate I, 5)

The association is dominated by foraminifers consisting of Lockhartia, Keramosphaera, Sphaer- ogypsina and Miliolidae, etc. Calcareous algae include Dasycladaceae (Trinocladus, Cymopolia, Dis- socladella), Udoteaceae (Halimeda, Ovulites) and Acetabulariaceae (Clypeina). Other biota are echinoderms, bivalves, gastropods and ostracodes.

According to (Reiss and Hottinger, 1984: 283), Daviesina and Lockhartia are typical of shallow subtidal environments with higher water agitation during the middle Paleocene and Early Eocene. The occurrence of Lockhartia and the comparatively high biota diversity suggest that the sedimentary environment of BF-21 is an open marine platform (FZ6A, Fig. 4) with a good water circulation.

4.3.17. BF-22 Nummulites-Alveolina (Plate III, 4; Plate V 1, 3)

The biofacies is dominated by the abundant larger foraminifers (Nummulites, Alveolina, Orbitolites, As- silina and Discocyclina). The small-sized foraminifers occur in the space among the larger foraminifers. They are Miliolidae, Vemeuilinidae, Textularridae, Lockhartia and a few planktic foraminifers. Other minor biota include echinoderms, bivalves, ostracodes, bryozoans and calcareous algae (Lithoporella, Distichoplax biserials).

The dominant nummulitids and alveolinids lived during the middle Eocene as habitants of soft bot-

Plate IV 1. Halimeda, BE1 8, Section M, Sample Mrl 1, ?Danian. x 20. 2. Alveolina and Orbitolites, BF-17. Section F. Sample F16, Thanetian. x 10. 3. Miliolidae and Dasycladaceae. BF-20. Section M, Sample Mr42, ?Danian. x20. 4. Indopolia. BF-20. Section M. Sample Mr32, ?Danian. x20.

16 Zhou Zhicheng et ~1. /Marine Micropuleontology 32 (1997) 3-29


toms and/or areas with low water-energy in depth of 50-90 m (Reiss and Hottinger, 1984, p. 283). The depositional environment of BF-22 can be interpreted as an open marine platform (FZ6A, Fig. 4) or a deeper part of a homocline ramp.

4.3.18. BF-23 bivalves-gastropods The biofacies is characterized by autochthonous

bivalves (oysters, Cardium, Mytilus, Nucula) and gastropods. There are some redeposited foraminifers (Alveolina, Nummulites, Orbitolites, miliolids), echinoderms and bryozoans, etc. The fossils coex- ist with the radiaLfibrous ooids which are typical indicators of hypersaline coastal pond environments (Fliigel, 1982, p. 147). Another outstanding diagenetic feature is the high number of authigenic bipyra- midal quartz crystals often occurring in the centre of the ooids.

As above, the biofacies documents the last marine ingressions into a restricted, slightly hypersaline cutoff lagoon or coastal pond (FZ8, Fig. 4).

4.4. Redeposited biofacies

4.4.1. BF-24 redeposited biofacies (Plate v 2) The redeposition, caused mostly by turbidite or

debris flow, led to the mixture of different biota derived from variant environments and variant strata. Sometimes, two different kinds of pebbles accumu- lated together. One came from the Zongshan Forma- tion of Maastrichtian age and formed in the shallow water environments. The others derived from pelagic deposits of Santonian-Campanian age. This typical biofacies in genesis is termed redeposited biofacies.

The biofacies is characterized by having poor sorting, roundness and chaotic texture and includes hemipelagic or pelagic biota and benthic biota.

According to the genetic features of this biofacies the sedimentation took place mostly in the distal continental slope (FZ2, Fig. 4) or the proximal continental slope (FZ4, Fig. 4).

5. Ichnofacies

Trace fossils are regarded as a useful tool for interpreting the environmental factors such as bathymetry, sedimentation rate, oxygenation, substrates, energy level and salinity. The evolution of sedimentary environments and paleogeographic as- pects in southern Tibet during the period from Creta- ceous to Paleogene led not only to changes of biofa- ties, but also to changes of ichnofacies. Variant trace fossil assemblages have been found in the various kinds of deposits from different strata. Every trace- fossil assemblage is related to a certain sedimentary environment. According to Seilacher (1967), Frey and Seilacher (1980) and Ekdale et al. (1984), the trace fossil assemblages can be attributed to Nereites, Zoophycos, Cruziana and Skolithos ichnofacies.

5.1. Nereites ichnofacies

The trace fossils Phycosiphon sp. (Plate VI, 4) and Palaeophycos tubularis? have been found in the siltstones in the lower part of Gamba Group of Neoco- mian age. A similar trace fossil assemblage occurs in the muddy siltstones and marls of Formation 2 of the Gamba Group aged middle-late Albian. The trace fossil assemblage includes Megagrapton? sp., Phy- cosiphon sp., Taenidium serpentinum (Plate VI, l), Planolites montanus, P. beverleyensis, Chondrites sp. and Palaeophycos tubularis? Most of the trace fossils of the assemblage are fodinichnia. Mega- grapton has been found in the flysch deposits of the Polish Carpathians (Ksiazkiewicz, 1970). Taeni- dium, Phycosiphon, Chondrites are main members of Nereites ichnofacies in the Ouachita Geosyncline (Chamberlain, 1978). The above mentioned trace fossil assemblages found in the Gamba Group can be assigned to the Nereites ichnofacies.

Nereites ichnofacies is usually considered to be the trace fossil assemblage produced on the soft sub- strate in the deep sea environment. The deposits in

Plate V 1. Nummulites and Alveolina. BF-22. Section M, Sample Mr84, Eocene. x 10. 2. Redeposited calcispheres, Textuluria and Omphalocyclus fragments, BF-24, Section L, Sample Lf 122, Maastrichtian-Paleocene. x 20. 3. Nummulites, Assilina and Discocyclina, BF-22. Entong Section, Sample 79DN16, Eocene. x 10. 4. Zoophycos sp., Zoophycos ichnofacies, Zoophycos-Chondrites suhichnofacies. Section At, Sample AT22, Campanian. 5. Chondrites sp. in thin section. Section A, Sample Ar7, Albian. x.5.


which the Nereites ichnofacies occurs contain ammonites, belemnites and planktic foraminifers. Com- pared with the facies model in this paper (Figs. 2 and 3), the appropriate sedimentary environment that the Nereites ichnofacies reflects is an open marine basin below lower wave base.

5.2. Zoophycos ichnofacies

The trace fossils Zoophycos (Plate V, 4), Chon- drites, Thalassinoides sp. and Planolites occur in Limestone 1 and Calcareous Marl 1 of the Zong- shan Formation in Gamba and Tiina. The trace fossil assemblage can be referred to the Zoophycos ichnofacies. An interesting phenomenon has been found in Section At in Tiina. The trace fossil assemblage in the limestones with lower terrigenous influx is strikingly different from that in the marlstones with higher terrigenous input. The trace fossil assemblage in limestones consists mainly of trace fossils of the upper tier which include Thalassinoides, Planolites and Phycodes? (Pla.te VI, 6). This trace fossil assemblage is characterized by its high diversity and high abundance. By contrast to this, the trace fossil assemblage in the marlstones contains mainly trace fossils of the deeper tier. They are Zoophycos and Chon- drites. The trace fossil assemblage is characterized by lower diversity and lower abundance. The two trace fossil assemblages alternate in the vertical section (Fig. 4). Brom.ley and Ekdale (1984) described the oxygen-controlled tiering relation of trace fossils in selected examples of Mesozoic marine strata that represent oxic to anoxic depositional environments. When the oxygen content of bottom water decreases, the trace fossils Planolites and Thalassi- noides disappear from sections first and Chondrites and Zoophycos disalppear last. Therefore, Chondrites and Zoophycos are good indicators of dysaerobic or anaerobic conditions within the sedimentary environ-

ments. The Thalassinoides-Planolites assemblage is produced in aerobic environment. The alternation of the two trace fossil assemblages in section At was caused by the changes of oxygen content in bottom water. The Zoophycos ichnofacies in Tiina can be subdivided into Thalassinoides-Planolites subichnofacies and Zoophycos-Chondrites subichnofacies which were representative of the aerobic and dysaerobic conditions of the original depositional environments, respectively.

According to Frey and Pemberton (1985, p. 99), the Zoophycos ichnofacies is typically portrayed as an intermediary between the Cruziana and Nere- ites ichnofacies, at a position corresponding more or less to the continental slope. The deposits bearing Zoophycos ichnofacies are rich in hemipelagic and pelagic organisms like planktic foraminifers, calcispheres and calpionellids. The graded bedding produced by distal turbidites and the bioturbated stmc- tures are present in the deposits. The sedimentary environments represented by Zoophycos ichnofacies in Tiina and Gamba correspond to shelf margin and open sea shelf environments reflected by BF-2 and BF-3.

5.3. Cruziana ichnofacies

Abundant trace fossils of this group have been found mainly in the siltstones, fine-grained sandstones of Formation 2 of the Gamba Group. They include Gyrochorte comosa, Syringomorpha hori- zontalis, Fucusopsis sp., Teichichnus sp. (Plate VI, 5), Chondrites sp. (Plate V, 5; Plate VI, 3) and be- long to fugichnia. The trace fossil assemblage is characterized by high diversity and is referred to the Cruziana ichnofacies (Fig. 5).

Formation 2 of the Gamba Group contains abundant inorganic structures including cross-bedding, storm layers, slumping structures, channel deposits,

Plate VI 1. Taenidium serpentinum, Nereires ichnofacies. Section A, Sample Ar63, late Albian, polished section. x 1.5. 2. Arenicolites sp., Skolithos ichnofacies. Section D, Jidula Formation, Maastrichtian-Early Paleocene, x0.43. 3. Chondn’res sp., Tiina, late Albian? x0.67. 4. Phycosiphon sp., Nereites ichnofacies. Section K, Sample Krl, Neocomian. x 1.3. 5. Teichichnus sp., Cruziana ichnofacies. Section A, Sample Ar13, middle-late Albian, polished section. x 1. 6. Thalassinoides, Planolites and Phycodes? assemblage, Zoophycos facies, Thalassinoides-Planolites subichnofacies. Section At, Sample At1 6, Campanian. x 1.


Fig. 5. The distribution of biofacies and ichnofacies in Section At, Tiina. Th. = Z’halassinoides; P. = Planolites; Z. = Zoophycos; Ch. Chondrites.


sandy neptunic dikes, graded bedding and Bouma sequences. The sedimentary structures show that the deposits have been influenced by storms and turbidites. Commonly, Cruziana ichnofacies is found on open continental shelves. The Cruziana ichnofa- ties of the Gamba Group is produced on an open sea shelf, shelf margin where the activities of storm waves and turbidites reached.

5.4. Skolithos ichnc$acies

When geologists examine to the sandstones of the Jidula Formation in Gamba and Tuna, they will be impressed by the abundant, densely packed trace fossils in the sandstones. The vertical trace fossils include Arenicolites sp. (Plate VI, 2), Skolithos Zin- earis and Skolithos !jp., and are referred to the typical Skolithos ichnofacies.

As Frey and Pemberton (1984) and Read- ing (1986) indicated, the environments which the Skolithos ichnofacies represents are characterized by shifting substrates, moderate- to high-energy conditions and episodic (erosion or deposition. The environments include the foreshore and shoreface zones of beaches, bars and spits (Howard, 1972, 1975). The fine-medium g,rained sandstones of the Jidula Formation have medium to large cross-beddings at the lower part (Mernber 1) and medium to small di- mensional cross-beddings at the upper part (Member 3). The Skolithos ichnofacies and the physical sedimentary structures indicate that the sandstones were deposited in the high-energy environments of sand- barriers in subtidal shoreface zones and intertidal foreshore zones.

6. Sedimentary history

The preceding chapters describe the important Cretaceous-Paleogene biofacies and ichnofacies in Tuna, Gamba and Tingri and briefly indicate their environmental significance. The evolution of biofa- ties and ichnofacies in the vertical sections is important in understanding and reconstructing the marine sedimentary history from Cretaceous to Paleogene in southern Tibet (Figs. 5-9). The main facies zones are summarized in Fig. 2 (according to Willems, 1993). In this chapter, the outline of the sedimentary history is generalized.

6.1. Tiina, Gamba areas

6.1. I. Neocomian-Albian (basin environments) During the Neocomian-Albian (formations 1,2 of

the Gamba Group), basin environments were dominant. Based on the features of biofacies, ichnofacies and petrology, two different basin types can be distinguished. One is the euxinic basin and the other is an open marine basin. The sedimentation may be related to the global oceanic anoxic event from Barremian to Albian (Schlager and Jenkyns, 1976; Arthur and Schlager, 1979).

The depositional conditions in the open marine basin differed from those of the euxinic basin by having a good water circulation and higher oxygen content. The deposits comprise siltstone, fine- grained sandstone and marlstones in which the nektonic and planktic fossils (ammonites, planktic foraminifers), benthic fossils (gastropods, bivalves) and biogenic structures (Nereites ichnofacies) can be found. The sedimentation was also influenced by distal turbidites. A short regression took place in the Albian. The Cruziana ichnofacies of Formation 2 of the Gamba Group indicates a shelf margin and open sea shelf environment.

6.1.2. Cenomanian-Campanian (shelfmargin, open sea shelf and platform slope environments)

The regression began in the Cenomanian and continued to the Campanian leading to the transition of the environments from the basin to the prevailing shelf margin and open sea shelf. Carbonate deposits play an important role in the sedimentation of these environments. The deposits comprise mostly marlstones and limestones (Formation 3 of the Gamba Group and the lower part of the Zongshan Forma- tion). The biofacies are mainly composed of BF-2 and BF-3. The Zoophycos ichnofacies at the lower part of the Zongshan Formation can be divided into the Thalassinoides-Planolites subichnofacies and the Zoophycos-Chondrites subichnofacies. The alternation of the two subichnofacies in the vertical section (Fig. 5) was caused by the episodic fluc- tuating of aerobic and disaerobic conditions at the sea floor. During the very late Campanian, the sedimentation took place mainly in the platform slope located in the transitional facies zone between the open sea shelf and carbonate platform because of

Zhou Zhicheng et al. /Marine Micropaleontology 32 (1997) 3-29

VJ T

-Y

I . . . . I 0 Fig. 6. The distribution of biofacies and ichnofacies in sections A, K, Gamba.

Ichnofacies

CTUZiaSU N eTe&?es

ichnofacies ichnofacies

I

the further regression. The biota in this environment represented by BF-6 are characterized by the coexistence of benthic organisms and the pelagic or hemipelagic organisms.

6.1.3. Maastrichtian (the development of a carbonate platfomt)

During the Maastrichtian, a carbonate platform was established and developed in Tiina and Gamba. The biota on the carbonate platform are characterized by the predominant benthic organisms with high


Fig. 7. The distribution of biofacies and ichnofacies in sections D, Zp, Zm, F, Gamba.

diversity. In the early and middle Maastrichtian, the smaller foraminifers Miliolidae and the larger foraminifers Orbitoides, Omphalocyclus are important rock-forming members, and in the late Maastrichtian the calcareous a1ga.e play a more important role in the composition of biota. The bioclasts usually with micrite envelopes show the depositional features in the shallow water environments. In general, the car-

bonate platform went ahead landwards from the early Maastrichtian to the late Maastrichtian. Two stages, platform margin and open marine platform, can be distinguished. The platform margin is represented by high-energy shoals (BF-7, BF-8, BF-9) and rudist biostromes (BF-10). In the late Maastrichtian, sedimentary environments were deposited in the open marine platform including the carbonate platform

24 Zhou Zhicheng et al. /Marine Micropaleontology 32 (I 997) 3-29

~1 Tingri .x d Section I

3

-

M

-

LOO

8OC

500

400

100

0 -

3iofacies

Fig. 8. The distribution of biofacies in Section L, Tingri

and tenigenously influenced platform conditions. The biofacies (BF-11) formed in the carbonate platform consists mainly of Omphalocyclus and different kinds of calcareous algae (Corallinaceae, Dasy- cladaceae and Udoteaceae). Afterwards, the Coral- linaceae (BF-12) forming the rhodoids prevailed in the terrigenously influenced platform.

61.4. MaastrichtianlPaleocene boundary (sandstone accretions and lagoonal environment)

From the end of the Maastrichtian to the beginning of the Paleocene, the carbonate dominant sedimentation terminated because of the large quantities of terrigenous quartz sand input. The sand barriers

were deposited in the intertidal foreshore zone and the subtidal shoreface zone. The Skolithos ichnofa- ties formed under the shifting substrates; moderate- to high-energy and episodic erosion or deposition conditions flourished in the sandstones of the Jidula Formation. The sandstones are separated by a black pyrite-bearing limestone which contains a lower diversity biota including Ovulites, ostracodes, dasycladaceans, rudists and smaller foraminifers. The limestones indicate a nearshore lagoonal environment.

6.1 S. Middle Paleocene (open marine platform) The next transgression commenced at the begin-

ning of the middle Paleocene and resulted in the accumulation of carbonate deposits formed in the open marine platform. The marlstones and limestones of Member 1 of the Zongpu Formation (Fig. 6) are superimposed on the sandstones of the Jidula Formation. In the well circulated environment, calcareous algae flourished again; they comprise Dasycladaceae, Acetabulariaceae, Coralli- naceae and Udoteaceae. The foraminifers Lockhartia and Verneuilina occupy an important position in the composition of the benthic foraminifers. The BF-14 and BF-1.5 are the representative biofacies during this period.

6.1.6. Thanetian (open sea shelf and open marine platform environments)

The transgression may reach a climax in the Thanetian when the sedimentary processes shifted from the open marine platform to the open sea shelf. The sedimentary processes are indicated by BF- 16 in which the larger foraminifers Miscellanea, Ranikothalia and Operculina occur. The transgression only lasted a short time. From the transition during the Thanetian, the sedimentary processes returned from the open sea shelf to open marine platform again. The sedimentary environment is reflected by the BF- 17 in which the larger foraminifers Alveolina and Orbitolites are dominant.

6.1.7. Latest Thanetian (jinal stages of marine sedimentation: cutoff lagoons and coastal ponds)

During the latest Thanetian, the continued regression of the sea resulted in the end of normal marine sedimentation. Sedimentary processes


M

-

40 5.

Of

j0

1

Biofacies

LEGEND

E3 limestone

Ia nodular limestone

- - hJ_rl - marly limestone --

El dolomite

I, I N

1 marl

I I - L calcareous marl A_

El claystone

. . . . r-l siltstone . . . . . . .

. . . l-l .__ . . . fine - grained sandstone

. . I I -. ._ medium - gr&ed s&stone

coarse-grained sandstone

..___ _

cl .-..__ sandstone .._.. .

i-l Q septarium

PI thrusting

zig. 9. The distribution of biofacies in Section M, Tingri


shifted from marginal marine depositional environ- upper half, whereas the sandstones of the Jidula For- ments, tidal flats and coastal ponds into cutoff hy- mation (Early Paleocene) developed from seaward persaline lagoons. As a result, the limestones of the prograding delta plains (Willems, 1993, p. 127). The Zongpu Formation were replaced by the marlstones sediments in the various evolutionary stages of the and clay/siltstones of the Zongpubei Formation and paleo-continental slope are different. They originated the biofacies rich in stenohaline organisms, espe- mainly from turbidite and debris flow sedimentation cially the larger foraminifers were replaced by the as well as normal sedimentation. The typical biofa- BF-23 containing mainly autochthonous euryhaline ties that resulted from the resedimentation can be bivalves and gastropods. attributed to the redeposited biofacies (BF-24).

As described above, two main cycles of transgression and regression of the sea took place in Tuna and Gamba from the Cretaceous to Paleogene. The first cycle lasted from the Neocomian to the Maas- trichtian/Paleocene boundary. The second one lasted from the middle Paleocene to the end of the Thane- tian. The scope of the second transgression was much smaller than that of the first one. The marine sedimentary history ended at the Paleocene/Eocene boundary.

6.2.3. ?Danian (carbonate platform)

6.2. Tingri area

6.2.1. Late Albian-early Maastrichtian (open marine basin, basin margin and open sea shelf environments)

The sedimentary processes from the late Albian- Campanian occurred mainly in open marine basin, shelf margin and open sea shelf environments. The biota are characterized by pelagic and hemipelagic organisms (planktic foraminifers, filaments, calcispheres, radiolarians and spicules). The biofacies are represented by BF-2, BF-3 and BF-4 (Fig. 8). The euxinic deposits found in Gamba from Neocomian to Albian have not been found in Tingri area during the same period. The basin environments in Tingri area had the better water circulation than that in the Gamba area.

The termination of the strong terrigenous material input and the beginning of the transgression led to the establishment of a stable carbonate platform in Tingri during ?Danian times. The deposits of the carbonate platform began with the ooid grainstone deposited in the high-energy shoal at the platform margin and the wackestone, packstone, floatstone and rudstone containing Halimeda, echinoderms and smaller foraminifers, Dasycladaceae, Melobesieae, formed in the open marine platform. Afterwards, rhodolite bars and patch reefs were produced on the open marine platform. The lagoons cutoff from the open sea by the rhodolite bars and patch reefs were formed simultaneously. The BF-20 is indicative of this kind of low-energy environment. With the increase in the transgression, the sedimentary processes shifted from the rodolite bars, patch reefs and lagoonal environments to the open marine platform. The changes of sedimentary environments resulted in the growth of the calcareous algae and foraminifers especially of Dasycladaceae and Lockhartia (BF-14).

6.2.4. Thanetian-Lutetian (diversification of larger foraminifers)

6.2.2. Middle Maastrichtian-Early Paleocene (continental slope)

While a carbonate platform was establishing in the Gamba area during the Maastrichtian, a continental slope was developing in Tingri during the time from the middle Maastrichtian to the Early Pale- ocene. According to this model, the sediments of the Zhepure Shanpo Formation (from the middle Maas- trichtian to the Early Paleocene) represent deposits of a lower fan in the lower half, more of a mid-fan in the

A stable, carbonate platform only slightly differentiated morphologically (Read, 1982) is established during the period from the Thanetian to Lutetian. The sedimentary processes took place on an open marine platform for most of the time. A short transgression occurred in the middle and late Thanetian which resulted in the shifting of the sedimentary environments from the open marine platform to the open sea shelf. Consequently, the biota consisting mainly of the larger foraminifers Lockhartia and Keramosphaera (BF-21) were replaced by biota consisting mainly of larger foraminifers Miscellanea

Zhou Zhicheng et al./Marine Micropaleontology 32 (1997) 3-29 27

and Ranikothalia (BF-16). The sedimentary environments returned to the open marine platform from Thanetian to Luteti.an. The biota are characterized by the very large Nummulites, Alveolina, Orbitolites, Assilina and Discocyclina (BF-22).

Because the non-marine Zongpubei Formation has not been found in Tingri, the marine sedimentary history there ended in the Lutetian according to the youngest deposits of Lutetian limestones.

7. Conclusions

The understanding of the sedimentary environments and history of the Cretaceous and Paleogene in the Tuna, Gamba, and Tingri areas of southern Tibet is based on the detailed studies of biofacies and ichnofacies.

(1) Two major transgression/regression cycles can be distinguished from the Early Cretaceous to the Paleogene in the Gamba area. The first lasted from the Neocomian to the Cretaceous/Paleogene boundary and the second from the middle Paleocene to the Thanetian.

(2) The sedimentation processes in the Gamba area from Neocomian to late Albian took place briefly in basin environments from which two types, an euxinic basin and an open marine basin, can be recognized by the biofacies and ichnofacies. The former is reflected by the euxinic biofacies and litho- facies, while the latter contains ammonites biofacies and Nereites ichnolgcies. The Cruziana ichnofacies can be found from both the shelf margin and the open sea shelf.

(3) Calcispheres (calcareous dinoflagellates) flourished in the outer shelf environments from the Santonian to the Campanian. The Zoophycos ichnofacies occurred in the zones from the open sea shelf to the platform margin from the Campanian to the lower Maastrichtian; this can be further divided into two subichnofacies: Thalassinoides-Planolites subichnofacies and Zoophycos-Chondrites subichnofacies. Their alternation in the vertical section was caused by the fluctuation of the oxygen content on the sea floor. The Th.-P. subichnofacies was produced under aerobic conditions, and the Z.-Ch. subichnofacies under disaerobic conditions. The transitional zone between the open sea shelf and the carbonate platform is also indicated by

the Goupillaudina-Sulcoperculina biofacies which is characterized by the coexistence of the benthic biota and the pelagic biota.

(4) Not until the Maastrichtian was a stable carbonate platform established in the Gamba area. The carbonate platform went through several stages: higher-energetic shoals and rudist biostromes, open marine platform and the terrigenously influenced platforms. The benthic foraminifers (Orbitoides, Omphalocyclus and Miliolidae), calcareous algae (Corallinaceae, Dasycladaceae and Udoteaceae), and rudists (hippuritids and radiolitids) are the most important biota in these environments. The Coralli- naceans (Archaeolithothamnium, Lithothamnium and Lithophyllum) encrusted other bioclasts to form the rhodoids, which occurred in the nearshore marine platform.

(5) The occurrence of the thick-bedded Jidula sandstones from the late Maastrichtian to the Early Paleocene marked the end of the first regressive cycle in the Gamba area. The typical Skolithos ichnofacies, widely found in the sandstones, indicates nearshore environments in which sand barriers were built. Black limestones bearing an Ovulites- ostracodes biofacies are intercalated in the Jidula sandstones. They were produced in lagoonal environments with stagnating water circulation.

(6) In Tingri, the sedimentation processes during the time from the Vraconian to the Campanian were similar to those of Gamba. After the diastema of early Maastrichtian, heavy redeposition took place on a paleo-continental slope in the Tingri area.

(7) At the beginning of the middle Paleocene, a major transgression took place in the areas of Gamba and Tingri. In the Tingri area, a stable carbonate platform was formed. First, Halimedu-echinoderms biofacies was produced in the open marine platform and this was followed by the biofacies of Pseu- dolithothamnium album and corals produced in the rhodolite bars and patch reefs in the nearshore environments. The larger foraminifers, Miscellanea, Ranikothalia, Opeculina, Discocyclina, and rotaliids expanded in both Gamba and Tingri during the Thanetian.

(8) The termination of the marine sedimentary history ended in the Thanetian in the Gamba area, which was earlier than in the Tingri area. There, larger foraminifers Nummulites, Assilina and Aster-


ocyclina spread widely into the Lutetian which was the last stage of the marine sedimentation in Tingri.

Acknowledgements

We thank the German Science Foundation (Wi 725/5), the Max-Planck-Gesellschaft, and the Academia Sinica for their financial support of the Ti- bet Expeditions. We thank Prof. D.G. Jenkins, Prof. W. Berggren and Prof. Wang Pinxian for their com- ments and suggestions leading to the improvement of this paper.

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