DISSOLUTION PIPES INNORTHERN PUERTO RICO: AN EXHUMED PALEOKARST

Joyce Lundberg' and Bruce E. Taggarf

'Department ofGeography, Carleton University, Ottawa, Ontario Kl S5B6, CANADA2US Geological Survey, GSA Center, 651 Federal Drive,Suite 400-15, Guaynabo, Puerto-Rico, 00965 USA

ABSTRACT: Late Quaternary pipe- or wen-like paleokarst features are being exhumed and modified by modem coastal processes alongthe north-western and northern coasts of Puerto Rico. These features are cigar-shaped tubes dissolved into host rock, with depths up to 4 rn,and widths of -0.5 m. They can be so densely packed that much of the original deposit has been removed. Most contain evidence of a fewmillimeters thick calcrete lining, consisting of micrite laminae, and a zone of indurated rock up to several centimeters thick of micrite andmicrospar. Many pipes are filled with insoluble material similar in appearance to the insolubles of the host rock but more concentrated, andaugmented by material which resembles terra-rossa, At one site the pipes have retained this primary fill material, now somewhat cemented.At the other site the primary fill material, probably sand rather than rerra-rossa, was completely removed, the pipes re-filled with marinedebris and the whole complex cemented. Some pipes show more than one cycle of filling, emptying and re-filling, and some areas showmore than one phase of pipe formation.

The pipes formed in the vadose zone, in poorly lithified, coarse-grained, Late Quaternary sandy limestones, by dissolution and re­precipitation along focused flow paths in a climatic regime with rain and strong evaporation. They may have formed within a few thousandyears of host rock emplacement.

INTRODUCTION

Pipe- or well- like features which appear to bedissolutional in origin are described in the literature from awide variety of locations, in different materials, and undermany different names. The most common seem to be thosedeveloped in the surface 10 m of tropical carbonates andgenerally called "solution pipes". This paper reviews thereported occurrence of these features, here more accuratelytermed"dissolution pipes";describes pipes of the north andwest coasts of PuertoRico; and discusses possible modes ofpipeformation.

The term"solution pipe" has been used somewhatindiscriminately in the literature for dissolution features ofgreatly differing scalesand forms (eg,Kirkaldy 1950; Link1967; Lefebvre et al. 1968; Thorezet al. 1971; Brunsden etal. 1976; De Bruijin1983; Rasmussen and Neumann 1988;Webb 1994). Many of the "solution pipes" in the literaturecould adequately be described by terms already establishedfor karren forms of purely dissolutional origin such as"dissolutional wells", "cavernous karren"or"epikarst"(Fordand Williams 1992). An example is the clay-filled sinuouspassages upto40 cmwideand 1 to2 mlongandopening intohorizontal bedding planepassages, developed in the OoliticLimestone oftheCarboniferous ofSouth Wales (Wright 1983,1988). These are paleo-epikarst passages rather thandissolution pipes.

It is suggested that the term "dissolution pipe" bereserved for the small scale (0.25-1 m wide, 1-4 m deep)dissolution features occurring in poorly lithified, relativelycoarse-grainedrocks and usuallyassociated with calcrete, Day(1928) describes classic dissolution pipesin dunesandstonesof Syria. The rockis perforated withnumerous vertical pipesfilledwith unconsolidated red clay.

Carbonates and EvaporiUs, v. 10, no. 2, 1995, p. 171-183

''Theseholesarealways vertical, from 10to20inches[-25 to50 em] in diameter, and from 5 to 15 feet[-1.5 to 3 m] deep,spreading out funnel-like at the surface and tapering like acigarat thebottom. Themostremarkable features ofthesearetheir regularity and the smoothness of their walls. ... Thehorizontal section approximates closely toa circle."

Similarfeatures are often called"palmetto stumps"since Livingston (1944) postulated that pipes formed wherecalcareous molds formed aroundthe trunksoftreesburiedbyadvancing dunes. This theory was rapidlydiscounted forlackofsupporting evidence but the termhas survived (eg, Carewand Mylroie 1994). Other terms include "chimneys"(Fairbridge 1950), "soilroots"(Bretz 1960), "karstpotholes"or "solution pits" (Shinn and Lidz 1988, Mylroie 1988),"makondos" (Brink andPartridge 1980), "organpipes"(Priceand Verhoef 1988). Link (1967) uses the term ''well'' withadescriptor of the natureof the fillmaterial; eg, ''boulder/clay/sand/scree- well",

Dissolution pipes occur both in the contemporarylandscape and in the geological record. Their presence andtheirstructure mayelucidate the natureof hardgrounds in thegeological record (see discussion in Rose 1970 of theimportance of, and difficulties in, identifying hardgrounds).They may contribute more information about the exposurehistory than is apparentfrom thenon-piped exposure surfacealone. Multiphase piping mayindicatedetails ofevents duringexposure or hiatuses in deposition whereno other evidencemay exist for example, Walkden and Davies (1983) foundevidence forfour phases oferosion ona singleerosion surfacebystudying the natureofpipesand fill.

Characteristics ofdissolution pipes

Thereis surprisingly littlevarietyof form in pipes.

DISSOLUTION PIPES IN NORfHERN PUERTO RICO

Pipewidths areusually30to60em(eg,30to60eminWesternAustralia, Fairbridge 1950; 30 em average in beachmaterialin Britain, West 1973; 30 to 40 cm in Zululand andMozambique, Coetzee 1975; 20to37eminBermuda, Herwitz1993). Depths are more variable: 3 m in Western Australia(Fairbridge 1950),2.2 m in Bermuda(Bretz 1960), up to 20m in Zululand and Mozambique (Coetzee 1975). Withwidth:depth ratios of 1:6 up to 1:50, pipesare of cylindroidform. Theyare commonly densely packed: Fairbridge (1950)reported pipes in Western Australia so densethat thereweremore pipe hollows than solid rock; pipes in eolianites inZululandand Mozambique cover about9.5% of the surface(Coetzee 1975) and in Bermuda up to 5% of the surface(Herwitz 1993). Pipesare generally vertical regardless of thedepositional dip of the strata in which theyoccur.

Generally pipes have an indurated lining withinduration continuingin the host rockbeneath the pipe, andcommonly contain a fill of paleosol or sand material, oftenlatercalcified. For example, thepipesofBermuda havea redclay fill (Bretz 1960), calcite casings 1-3 em thick, andcalcareous vegemorphs* (Herwitz 1993). However, onlysomeof the pipes of Zululand and Mozambique have a thinencrustation; some are filled with sand which in turn wascalcified; while some have a soft clay core and others haveCaco3 encrustations filling the pipes (Coetzee 1975). Manyof the indurated linings of the pipes have been etched outfrom the surrounding indurated rock to form an invertedtopography (Herwitz 1993; CarewandMylroie 1994). In somecases structures in the fill are concordant with structures inthe hostrock:Mylroie (1988) and Carewand Mylroie (1994)have called such pipes in San Salvador Island, Bahamas,"ghosttubes".

The climate from which most pipes have beenreported is tropical: eg,southern andwestern Australia (Baker1943; Fairbridge 1950; Baker 1958; Jennings 1968; Bird1970); Africa (Coetzee 1975; Brink and Partridge 1980);southern USA (Prouty and Lovejoy 1992); tropical islands(Livingston 1944; Bretz 1960; Adams 1983; Rasmussen andNeumann 1988; Herwitz 1993; Carew and Mylroie 1994).However, they also appear to form in cold but stronglyevaporative climates. For example, piping in carbonate­cemented siliciclastic beachdeposits in southern Britainmayhave fanned during periglacial conditions whilesolifluctiondeposits wereforming: the beach deposit wascompressed byoverlying solifluction deposits beforebeingcemented andpipedandsomeofthepipesarefilled with solifluction deposits (West1973,Ford 1984).

The lithology in which the majority of pipes haveformed is eolianite: eg, in Australia(Baker1943; Fairbridge1950; Baker 1958; Bird 1970), in Bermuda and Bahamas

* This term, representing any calcified vegetative traces, was in­troduced by Mylroie (1988) and is supported and defended inCarew and Mylroie (1995).

172

(Livingston 1944; Bretz 1960; Adams 1983; Herwitz 1993;Carew and Mylroie 1994), in Zululand and Mozambique(Coetzee 1975). Pipeshavealso formed in other lithologieswithhighprimarypermeability, suchas waterlaidcalcareoussandstones in Devon (Falcon 1929); beachdeposits in Britain(West 1973); chalk in Norfolk (Burnaby 1950); Devonianlimestone from Southern England (Brunsden et al. 1976);cave breccias in S. Africa (Brink and Partridge 1980); alithified Pleistocene Coquina deposit in Texas (Prouty andLovejoy 1992); Carboniferous oolitein SouthWales (Wright1983); and oolitic, bioclastic grainstoneof Mississippian agein Arkansis (Webb 1994).

Thegeologic timein whichmostpipeshaveformedis the Pleistocene, but, as the importance of paleokarst isbecoming understood, more are being reported from othertimes such as the Carboniferous, and the Cretaceous. Pipesare found, for example, in the Mississippian-Pennsylvanianunconformity of northwestern Arkansas (Webb 1994); inLower Carboniferous bioclasticcalcarenitesofAnglesey, NorthWales (Baughen andWalsh 1980; Walkden andDavies 1983);in Carboniferous Oolites of South Wales (Wright 1983); inCretaceous chalksof southern Englandand northernIreland(Burnaby 1950; Ford1984; SmithandMcAllister 1995).Pipeshave also been reported in Devonian limestone of southernEngland(Brunsden et al. 1976).

THE PUERTO RICO PIPES

Methods

Most of the observed pipe sites are on the northerncoastbetween Punta Corozo and Punta Maldonado (Fig. 1).Manyare in urbanor industrial areas of limitedaccess, andmanyarenowin thelittoralzone. Thesiteschosen fordetailedstudyare: Punta Higuero on the west coast; and East Island.PuntaSalinason the northcoast(Fig. 2). At bothsitesstudywaslimitedtopipesthatoccurseveral metersabove sea levelor greater to avoid complications from modern littoralprocesses.

Atlantic Oc~an

Sanjuan

PUERTO RICO

CaribbeanS<4

Figure 1. Location mapofPuerto Rico.

LUNDBERG ANDTAGGART

PUNTA SALINAS and EAST ISLAND

Isla de las Palomas

Ba"lade Too

Ensenadade Boca Vieja

0.25 Km

Contour Interval:1 m (up 105m), and 5 m

l\Figure 2. PuntaSalinas andEastIsland site.

-15 rnasl

1--50m-t

~-Beach calcarenite

Pipes were mapped, measured, and photographed.Hostrocks and pipe margins were sampled for further study.Thin sections were prepared from epoxy impregnated handspecimens. Unstained sections were examined andphotographed. Percentage cover wasestimatedusingthechartsin Fluegel (1982p. 247-257). Pointcountingof quartz grainabundance followed thetechnique outlinedin Fluegel(1982);the errorswere estimated using the methodof Plas and Tobi(1965). X-raydiffraction and Ion Microprobe analyses werecarried out at the University of Carleton, Earth SciencesDepartment. Carbonandoxygen isotopic analysis ofhostrockandindurated crustwasdoneat theIsotope Lab,University ofOttawa.

Results

The north coast shoreline of Puerto Rico in thevicinity ofPunta Salinasconsists ofPleistocene reefrockandcemented dunes with intervening beaches abutting onto analluvial plain (Kaye 1959; Williams 1965). The pipes occurin the cemented dunes and beach material exposed in theheadlands. The west coast shoreline around Punta Higiieroconsists of Pleistocene beach sands (terrigenous sands withCaC0

3clastsandcarbonatecement) ontopofearlier(possibly

Eocene) shellybeach deposits and Miocene limestone. Thepipesare in thecemented beach sandsof the headlands.

Iilled with

---15 masl

omas)--SOm--i

Figure 3. Diagrammatic viewofthe cliffsexposed to either sideofRam6nBay: To theeastnopipingis discernable in thecalcarenite unit, butthere isa trace ofpiping visible in the lower eolianite unit. To thewest thepipesareverywelldevelopedin the horizontally-bedded calcarenite unit.

173

Description: Evidence of pipes in this area is restricted to asmallarea which we havenamed CapeRam6n, on the westsideof Ram6n Bay (inset; Fig. 2). Theyoccurmainlyin thelower calcarenite unit(shown totherightin Fig.3). However,thereareindications ofsmallerpipesdeveloped in theeolianiteunit of the East sideofRam6nBay, exposed onlyin the cliffface and visible onlyfrom the opposite headland (shown totheleftin Fig.3). Thepipes on theeastsidemostlikely formedafter thedeposition ofthelowereolianite unit,probably at thesame time as the erosion surface and possible paleosol wereforming.Theyappeartobefilledwitheolianite material. Thepipeson the westside (Fig.s 4a,b,c) are filled withsandyredclay. Unfortunately, none of these units has been dated andthecoralboundstone which outcrops closetosealevelyieldedno dateable corals.

The pipesalong the west side of Ram6n Bay weremeasured and sampled. Depths, estimated from thefew pipesvisible in cliffexposures viewed from thenextheadland, are 1to 3 m and average 2 m. Widths, measured on the clifftop,rangedfrom 30 em up to 110 em with an average of 58 cm(25=64%,n=28).Estimates from sketch mapsdrawn in thefield indicate thatup to-10% of the cliff-top area is occupiedbypipes. Presumably thisproportion woulddecrease furtherinto the rock where the pipes taper in - so the proportion ofrocktaken upbypipes would onlybeabout5%. Manypipeswere within 1 m of another pipe (Fig.s 4b,c).

Lithology ofhostrock, pipe lining, fill material: (Fig. 5)

Host rock (Fig.s 6a.])): With -60% carbonate grains, 10%quartz, 15%voidspace and 5% cementthiscanbeclassifiedasa well sorted,sandy, bioclastic grainstone (Dunham 1962);

Figure 5. East Islandsite:Low magnification viewacrosspipeedge(PPL). Thehostrockbioclastic grainstone isshownto left. This gives way towards the middle to an induratedhaloof lower voidspace, microspar and micrite mottles. Inthecentre (slightly diagonal) are thecalcrete layers ofmicriteoutlining thepipe.To theright is thepipefill. Theblackspotsare artifacts.

DISSOLUTION PIPES IN NORTHERN PUERTO RICO

EastIslandSite.--

Figure 4. West side ofRamon Bay: (a) CapeRamon clifJ.1o.rge andnu­merous red clay-filledpipesin horizontally-bed­ded calcarenite unit; (b)population ofpipesonclifftop; (c) sketch mapofclifftoppopulation (plan view)showing the area coveredby pipes.

.:c

o

174

LUNDBERG AND TAGGART

a sandypackedbiosparite (Folk 1962); or a sandy allochemlimestone (Mount 1985). Subrounded bioclasts (mainly algalgrains)(-45%) andintraclasts (-15%), grainsize average530J.U11 (SD 429 urn), often have a micrite envelope; somearereplaced by microspar. Quartzgrains(average grainsize 200urn, SD 90 um) are angular to subangular, and often showpartiaI dissolution andreplacementbymicrospar. Isopachous,finelycrystallinecement indicatescementation undermeteoricphreaticconditions afterdissolutional modification ofgrains

Figure 7. EastIslandpipe - indurated halo1 emfrompipeedge(a) PPL, (b) XPL. Thisshould be compared with Figure5 to see the reduction in void space by precipitation ofmicrospar and micrite, and replacement of clasts withmicrospar.

andpartialmicritization ofbioclastic grainedges. Thesewereprobably beachsands.

Pipelining: Induration in theform ofreplacement ofbioclastsandinfilling ofvoidspacebymicrosparand micriteextends 1to 2 em into the hostrockfrom the pipemargin. Figure7a,bshows the resultant decrease in void space and decrease indefinition ofbioclasts.The pipe edge, althoughnot verydis­tinct, is marked by micritic calcrete laminae parallel to thepipe margin, 1 to 2 mm thick (Fig. 8). Allochems are notincorporated in the laminaeindicating that the laminaegrewdisplacively. The caliche laminae are broken, and anisopachous microspar cement linesthebreaks.InFigure9 theisopachous cementcan be seen to be composed of threelay­ers: a thin layer of micrite is sandwiched between twomicrospar layers (marked bythe arrow). The first microsparevent occurred during initial cementation of the sediment;micritization accompanied pipeformation; thefinal microsparcemented therock, pipeandpipefill. Ionmicroprobe analysisindicated a higherAIand Fecontentin the liningthan in thehostrockclasts orcement indicating a contribution from aero­sol (Rossinsky et al. 1992).

Fill material: Thepipesat East Islandare filledwitha sandy

175

Figure 6. General viewofthin section ofSample C EastIs­land host rock - sandy packed biosparite or bioclasticgrainstone. a) PPL; b) XPL. Note that the thin section hasmanyair bubbles.

Figure 8. East Islandpipe lining (PPL): micritic calcretelaminae, which do not incorporated allochems, parallel thepipe edge.

DISSOLUTION PIPES IN NORTHERN PUERTO RICO

Figure 9. East Island host rock: three layered isopachouscement (marked by the arrow): thefirst is a microspar; thesecond isa thinlayerofmicrite; the third is another layerofmicrospar.

red clay (Fig. 10) which is matrix supported. It consists of-30% quartz, 10% non-quartz siliciclastics, 50% red claymatrix, 15% void, and -5% unaltered bioclasts. The quartzgrainsof the fill have the sameaverage grain size and shape(sub-angular to sub-rounded) as those in the host rock, al­thoughtheyare slightlylesssorted. The average grain sizeofthequartzgrains in the hostrockis 0.18 mm (25=0.64%, n= 80)and ofthe fillquartzgrainsis 0.19mm (25 = 0.90%, n:: 80).However, the fill clearlyhas a greaterconcentration ofquartzgrains:pointcountingon hostrockquartzgrainsindi­catesthat theyconstitute 8.1±3%byvolume on thehostrock,and 29.4±4.4%of the fill.

X-raydiffraction analysis of the finerfraction of thefill produced peaks characteristic of quartz, calcite, and ka­oliniteand a broadlowpeakwhich mayrepresentamorphousironoxides (typical terra rossacharacteristics; Foos 1987). Acomparison run on the host rock showed that it consists ofcalciteand quartz with nokaolinite or feldspars. Thus, it ap­pears that the red clay fill has sometransported material andis not simply residue from host rock dissolution. The highlevels of Si, AI, Fe and Mg revealed by microprobe analysisindicate a probable contribution from aerosols (Rossinsky etal. 1992).

Punta Higiiero Site.--

Description: This population of pipesis exposed in a reced­ingcliffside(Fig.s lla,b) andin manyofthe blocks thathavefallen from the cliff (Fig.s 12a,b). Thesedifferfrom the Eastislandsitein that the hostmaterial is ofcoarsergrainandlesslithified, andthepipesarefilled withcoarse-grained, cementedmarinematerial.

The interpretive diagrams (Fig.s l lb and 12b) indi­cate manystagesofpartial or complete emptying and re-fill­ing of pipes. In Figure 12, pipeB shows indistinctstructureswhich mightbe vegemorph remnantsandotherswhich mightbeghostsedimentary structures inherited from the hostrock.

Figure 10. EastIslandpipefill (PPL): sandy red claymate­rial. matrix supported quartz grains ofsamesizeand shapeas hostrock.

Thereis no clear indication of a red clayfilling like the EastIsland site, but the lower parts of pipes A and B seem to bewellcalcified. Clearly the upperparts havebeenemptiedandre-filled withcoarsemarineclasts: coralfragments from PipeC havebeenradiometrically datedto oxygen isotope stage5e.PipeC was partiallyemptiedand filleda secondtime, againwithverycoarsemarineclasts. Then thewhole rock-pipe unitwas eroded,cross-bedded and sands deposited on top. Thatdeposit in tum was piped.

Not manypipeswereexposed at this site: the aver­age width ofthe6 pipesmeasured is 45 em, and the largestis60emat the top.Theytaperdownto 10em.Mostof thepipesare truncated, but theyrangefrom 1 to 2.5 m deep.The den­sityofpipesisestimated at -10% coverfromtheexposures inblocks which havefallenfrom the clifftop.

Lithology ofhostrock. pipe lining. fill material:

Hostrock (fig.s 13a,b): This is a poorlyIithified,grain-sup­portedrockwithvery high,mainlyintergranular, porosity (25­30%) and very poor sorting. The sparse cement (-5%) isisopachous microspar but there is some hint of meniscus ce­ments. Therockcontains 20%sub-angular quartzgrains (av­erage grain size 0.25 mm, 25 =97%, n :: 116); ;..,10% bigrounded quartzgrains(averagegrainsize0.49mm,25=60%,n :: 60); -20% large well rounded non-quartz siliciclastics(-0.9 mm ranging up to 5 mm); -5% sub-angular smallergrainsincluding -3% feldspar, 15-20% angularbioclasts (av­eragegrainsize0.33mm,25=85%, n :: 24);-5% carbonateclasts. LiketheEastIslandhostrock,thisisa sandygrainstone(Dunham 1962), sandy biosparite (Folk 1962), or sandyallochem limestone (Mount 1985). The near horizontal bed­ding and the coarse grain sizeindicates that thesewerealsoprobably beachsands.

Pipelining: The indurated zone extends about3 em into thehost rock from the pipe margin. In this zone void space isreduced to -5% in the0.5-3em zone,and to 0% in the outer

176

LUNDBERG ANDTAGGART

b

Figure 11. Punta Higeuro: Pipesfilled with marine debris. (a) PipeC is exposed in tilefallen block incentre ofphotograph. Partialfillingof otherpipes is visible in blocktothe right. Manyof the surfaces ex­posedarepipemolds; theflat facetisajointplane. PipesAandB showninFigure 12 are exposed in thecliffbehindthefigure. (b)Detailsofthemainpipearegivenin thediagram.Coral fragments from Pipe C havebeendatedto isotope stage Se.PipeC waspartially emptiedandfilledasecondtime. again withverycoarsemarine clasts. Thenthewhole rock­pipeunitwas eroded. andcross-bed­ded sands were deposited on top.Thisdeposit in turnwas piped.

Origin of thepipeform. --Thereare threepossible hypothesesthatcan explain theoriginof dissolution pipes. Theyare:

rosity at thePuntaHignero site,bothsitesare essentially thesame. Thepipeforms anddimensions aresimilarat both sites,andthepipemargins areindurated with microspar andmicrite.The significant differences may relate to the history of thepipes afterinitialformation. Thepipesconform to theclassic"solution pipe" described in the literature, although theydonot show obvious vegemorphs.

Discussion

0.5 mm. As in the East Island site,bioclastic grains are re­placedbymicrospar andmicrite (Fig.s 14a,b). Nomicrite lami­nae wereapparent (butit is conceivable that theeventwhichpartiallyemptiedandrefilled thepipewith coarsemarineclastscouldhavedestroyed the pipemargin).

Pipe fill: The majority of the fill material is clearly differentfrom thehostrockin termsofgrain size, lithology, and prob­ableage.Some of thefragments looklikeEocene lithoelasts,othersareclearlymarine bioclasts: oneAcroporapalmata coralfragment has beenradiometrically dated to isotope stage5e(Taggart 1992). There is no evidence to indicate that thesepipeswere everfilled byanyredclaydeposit. Thepresence oferosion surfaces within the fill indicates at leastonephaseofpartialemptying and refilling.

a) The syn-depositional hypothesis: The pipe formed as thedeposit was builtup, typically because a foreign form suchasa tree was in place, and has since decayed. Coetzee (1975)rejected the ideaof treeburialbymigrating dunes because oftheabsence ofvegemorphs orfossilized vegetation, distortionof the bedding, and lowerfossil soil horizons intowhichthetrees couldhaverooted. In addition the density of pipes was

Exceptfor the absence of calcrete laminae, and of toogreatin mostplaces. Herwitz (1993) convincinglyargued,red clayfill, and a hostrock of coarser grainand higher po- using densities, diameters, and sizes of pipes observed on

177

DISSOLUTION PIPES IN NORTHERN PUERTO RICO

b

\\,,

,\\,\ 7f\ E n I\ "'0 I., I

\ 8 \ ..... ' I

\ ~---+t,'\ ::J \ '\ \ "'0 .. J

\\_.....~.E ' .../

\ " '0'-- - 0

:2Figure 12. Punta Higuera site:Pipesexposed in cliff side: (a) photo, (b)diagram.

Figure 13. PuntaHiguera site: Host rocksandy biosparite. (a)PPL, and(b)XPL, show a poorlylithified, grain-supportedwith veryhighporosity, and isopachous microspar cement.

Bermudacomparedto mature trees, that the pipe populationdoes not represent the fossil remains of a contemporaneoustreecover, as forestecosystems donot showsuchgreatdensi­ties of mature trees. Similar arguments can beused to rejectthis theory for the pipesofPuertoRico.

b) The syn-lithification hnxMesis: The pipe formed by si­multaneous lithification and karstification: as thedepositwas

being lithified local concentrations of flow preventedlithification in pipesites. Jennings(1968) suggesteda simul­taneous development of all elements: sand lithification, pipeformation, and pipe filling. Biogenically acidified percola­tion water leaches surface layers of Caco3 and deposits itfurtherdown in the stratigraphic profilebuildinga resistantframework in the sands. This hypothesis explains many ofthefeatures ofpipesin general,and of thosein PuertoRicoin

178

LUNDBERG AND TAGGART

Figure 14. PuntaHiguera site: Pipe edge, (a)PPL and (b)XPL, shows thatmanyofthe carbonate clastsare no longerdistinct, and thatthevoidspace islargelyfilledwith microspar(the lessdensematerial ofthe groundmass) and micrite(themore dense amorphous material). This shouldbe comparedwithFigure 13a,b.

particular, and has been acceptedby many workers; for ex­ample, Wright (1983) attributes the pipes in CarboniferousOoliteof SouthWalesto local syngenetic karst. This hypoth­esis is the most likelyto explainpipe formation in deposits ofthe5e isotopesubstageas lithification is stillin theearlystagesnow.

c) The post-lithification h)l)Othesis: The pipe formed post­depositionallyand post-lithification, the mostcommonsitua­tionforkarst However, the distinction between pre- and post­lithification may be hard to pinpoint in Late Quaternaryde­posits, especially in poorly consolidated sands. West (1973)observed threegenerations ofcementation inPleistoceneraiseddunes of southern England; the first was an uneven sparrycalcite in the host rock, the secondwas the pipe lining (thepipesbeingformedby thedissolution oftheprimarycements),and the third wasa lithifactionofpipeandfill.Coetzee(1975)alsonotedthat manypipesare filledwithsandwhichhas alsobeenlithified.Clearlytheprocessoflithificationand pipefor­mation cannot be separated in time. In addition, in well-ce­mentedlimestones, jointingandbeddingsurfaces usuallypre­dominate as controls on dissolution forms; one of the com-

mon features of solution pipes is that they are never associ­ated with joints. This hypothesis is therefore probably notapplicable to most pipes.

A clear requirementfor pipe production is focusedvadose flow. Spencer (1981) argued that microtopographycreateslocalizeddeepwetting frontsunder smalldepressions.Coetzee (1975) also envisaged water collectingin some ini­tial irregularities, but expressed concern over the expectedslowrateof dissolution. AsHerwitz (1983)pointedout,pipesforming from an initialconcavity wouldhavea bowlshapeatthe top that elongatesdownwards secondarily. The extensionofthisideais thatyounger (smaller) pipesshouldhavea higherwidth:depth ratio than older(bigger) pipes.This doesnot ap­pear to be the case in PuertoRicowhere the pipes had a lowwidth:depth ratio with nowideningat the top.

An alternative involves vegetation. Jennings(1968)and Brink and Partridge (1980) suggestedthat water, acidi­fiedbyorganicacidsand biogenic CO

2, is concentrated around

stemsand roots, and thus focused beneathplants, promotinglocaldissolution and perhapssuccessive generationsof plantcolonization. Herwitz(1983)measuredflow ratesand chemi­cal properties of stemflow to support the argument that thepipes of Bermudaare causedby stemflow and that the highdensityof pipes are the resultof several generationsof trees.This processwouldproducea pipe shape that develops verti­callyfirst and an upperbowlshape later, the form mostcom­monlyobserved, both in Bermudaand in PuertoRico.

Althoughit is difficult to envision the maintenanceoffocused flowin sucha situation, somepipesappear to havedeveloped underneath a coverof sand Ford (1984)classifiedthepipes in the UpperCretaceous chalk of southernEnglandas "interstratalkarst" because theyare thought to havedevel­opedbeneathfluvial or marinesands.

Twofeatures of pipes discussed by Coetzee (1975)are the absenceof lateralcapillaryflow, and the restrictionofpipesto host rocksof highporosity. Byusing theoretical pen­etration distances in relation to capillary sizes and orienta­tions, Coetzee(1975) argued that solvententering a porousrock as a point source will penetratemainly in a verticaldi­rection.So, pipes tend to be limitedto porous rocks that are50% or moreCaC0

3•

Origin of indurated margin»- The indurated margins of thePuertoRicopipeshavemanyofthecharacteristicsofpedogeniccalicheor calcrete.Calcreteformation is generallyrelated toeluviation and illuviation ofcarbonates in soil, or in-situdis­solutionand re-precipitation of host limestone,and is gener­ally in closeassociation with vegetation (Carew and Mylroie1991; Rossinsky and Wanless 1992).The predominant cali­che fabric is one of clotted peloidal micrite with microsparchannelsand cracks. Microsparareas may show in-placere­placementof relictgrains and other microfabric. The calichecrust grades downwards or outwards into the original rock.

179

DISSOLUTION PIPES IN NORTHERN PUERrO RICO

Accessory fabricsare vegemorphs, pisolithsand poorlylami­nated micrite which may grow displacively. Plant roots con­tributeto the formationof alveolartexture, insitubrecciation,and micritization(Harrisonand Steinen 1978;Klappa 1980;Estaban and Klappa 1983; Webb 1994). The conditionsre­quired for calcreteformationare a hostrock of sufficient per­meability to allow significant capillary movement (Estabanand Klappa 1983); suitableclimaticconditionsof alternatingshortperiodsof rainfall and intenseevaporation (James1972;Webb1994);and a sourceofCaC03,which does not have tobefromthehostrock, as desertloess,or windblownsalt spray,mayprovideadequate sources(James 1972).

Although vegemorphs and peloidsare not a featureof thePuertoRicopipes, mostof the other featuresmentionedaboveare present, and, like those noted in Barbadoscalichecrusts (Harrison and Steinen 1978), multiple generationsofcement including alternations of both micrite and microsparcements were observed in the Puerto Rico pipe margins. Inaddition, the transformationof the grain-supported host rockto pedogenicmicrite-supported pipe margin matches Webb's(1994) observations on the calcreteof thePitkin limestones atthe Mississippian-Pennsylvanian unconformity. More nega­tivecarbonisotopevalues frompipe margins (wholerock)ascomparedto the host rocks (wholerock)indicatesa biogeniccomponent, and supports a pedogenicorigin. At the East is­land site, 013C

PDBvalues variedfrom -5.740/00 in host rock to­

7.38%0 in the pipe margin. For the Punta Higuerosite thoserespective valuesare -7.07%0 to -9.47%0. In additionthehigherAl and Fe concentrationsof the calcretelaminaecomparedtothe host rock bespeak an aerosol component. Therefore weaccepttheclassicpedogenicoriginforindurationofthePuertoRicopipes.

Origin offill material.-- Three hypotheses are envisionedtoexplain the nature of pipe fill material.The first hypothesis isthatpipeswereformedfirstand later filled. AlthoughJennings(1968) suggestedthat it was hard to envision such a processin poorlyconsolidatedsands,manyreportsimply that the ero­sion of the pipe was followed by its filling. Prasad (1983)assumed that red-elay filled pipes in limestone of Tanzaniamust haveformedas open pipes first, and then weresecond­arilyfilled. Wilson(1960)suggested that thepipesin Pliocenechalk pipes formed by dissolution first, then after sea levelrise the pipes filled with ferruginous sands, fine grits, iron­stones, and marine fossils. In the Puerto Rico pipes thetransitionarynature of the calcretized pipe margins (not al­waysa distinctedge, more oftena zoneof micrite induration)arguesagainst this origin.

The secondhypothesis is thatpipesare filledas theyare forming. The majority of pipes reported in the literatureare filled with clay-rich material often clearly related to apaleosol above. Herwitz(1993)suggests thatin Bermudapipesformedduring (or after) the formationof the terra-rossasoils(largelyderived fromatmospheric dustfrom cootinental sourceareas). West(1973) noted that the decalcified sand fill ofthe

pipes in beach sandstonefrom southern England formed asresidualmaterialfrom,and was concurrentto, thedissolutionof the host rockcement.In thePuertoRican pipesthe charac­ter of the pipe margin, the similarity of the clasts in the redclay fill to the clasts of the host rock, and the suggestion ofghostsedimentarystructureswhichextendfromthehostrockto the pipe fills, indicatesthat pipe ftlling was probablycon­temporaneous withpipeformation by dissolution of calciteinthe host rock under a soil or sedimentcover.

The third hypothesis is that pipeswerefilled,the fillerodedoutand thepipesre-filled.Baughenand Walsh(1980)report that in the paleokarstof north Wales, erosion deeplyscoured someof thepipes, whichwerethenfilled withcrinoidaldebris,leaving"no trace" of the original sand fill apparent inother pipes. The Punta Higueropipes appear to have had asimilar history of emptying and re-filling. Sea level rise re­moved much of the original fill (possibly sand, there is noevidence for a soilfill)and replacedit with marineclasts.Theabsenceof calcretelaminae in the pipe margins indicatesei­ther that the calcretization never progressed to the laminarform or that calcrete laminae did form but were destroyedduring the emptyingphase.

At Punta Higueroat least two phases of piping andfill indurationare indicatedbythe juxtapositionof two pipes,or the invasionof a later pipe into the fill of an earlier pipe,withoutthe reactivation of thefirstpipe. The sequence mighthave been: 1) concurrentdissolution and deposition to makepipes, lining, and fill, with sea level lower than the pipe ho­rizon; 2) sea levelrise, replacementof somefill and burial ofpipes with marine deposits (someclasts of which are of iso­topestage5e age);and 3) lithificationof hostrock,pipes,andfill. This sequencemust havebeen repeated to form the sec­ond phase ofpipes.Such complexity is not unique- Walkdenand Davies(1983)found evidencefor possiblyfivephasesoflithification and four erosive phases in a single paleokarstsurface of Anglesey, North Wales which representedonly aminorcycleboundarybetween successivedeposits. In thePuntaHigueropipe fill, the single radiometric date indicates thatthe firstpipe eventmust have happened(just?)beforeisotopestage5e sea level high. The top of the earlier pipe (PipeC inFigures 11 and 12) is at about 5 m above modern sea level.The last time sea levelwas definitelyhigher than present wasduring isotope substage5e (circa +6 m elevation). The hostrock is older than 5e; the initial pipes must be older than thecoral (but couldbe earlier 5e); the first emptying and fillingcycle occurred during the substage 5e or later. The secondgeneration of piped eolianites (overlying pipe C) are at about6 masl. They have alsobeen filled with marine debris. Thisrequiresa rapid oscillation ofsealevel:up to fillPipeC, downto allow eolianites and pipe formation, up again (althoughnot necessarily as high as +6 m) to fill the upper levelpipes.

It is possible that pipe formation, emptying and re­filling was a very rapid process (of the order of a few thou­sand years)and the two episodes of pipe filling with marine

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clastsrelatetotheisotopestage5edouble sea level peak (Moore1982; Aharon and Chappell 1986; Chappell and Shackleton1986; Mylroie et al 1991). Hearty and Kindler (1995) esti­mate the firstrise to havebeento +4 m and the second to +6m; the two rises are separated by a rubified protosol. If thesmaller, secondary pipesformed during thisprotosol episodeand the later higher sealevel emptied and filled them, thenthepipedepthsgivea possible lower limit(ofabout2 metres)to the rangeof the substage 5e double peak.

Analternative is that thefirsteventoccurred duringsubstage 5e and the second during substage 5c or 5a Thisscenario is less likelybecause sea level may not have beenhigh enough. Radtke and Grun (1990) suggest that sealevelpositions at 5a and 5c werebelow the level of the 5e maxi­mum.The 5a sealevel peakis generally shown at -10-20 mbelow presentsealevel (Moore 1992). However, thereis someevidence for a highersealevel, particularly in the coastal re­gions of USA: e.g. 7 m in California, 4 to 10 m AtlanticCoastal plain, and 3 to 10 m in South Carolina (Vacher andHearty 1989). Mylroie andCarew(1988) suggest thatthefreshwater lensonSan Salvador Island,Bahamas, had tobein the2 m to +7 m range between 85 and 70 ka BP. Vacher andHearty (1989), Heartyet al (1992), and Heartyand Kindler(1995) found evidence fora briefrise topresentsealeveldur­ing substage 5a in Bermuda (using aminostratigraphy on''widelyscattered, poorly preserved evidence").

CONCLUSIONS

The dissolution pipes developed along the north­western and northern coasts of Puerto Rico represent apaleokarst surface (now being exhumed by modem coastalprocesses). Therocktypes in which pipeshavedeveloped varyfrom fine-grained sandybioclastic eolianites (EastIsland), tocoarse-grainedsandybioclasticbeach material (PuntaHigiiero,Punta Salinas, East Island),toextremely coarse-grained reefrubble(PuntaCorozo).

Thepipesarecigar-shaped tubes withdepthsupto4m and widthsup to 1 m. Theycan be sodensely packed thatmuch of the original deposit has been removed. Theyoftenappearto haveformed in several generations in thesamerockunit,as indicated bylaterpipesintersecting those formed ear­lier.Theyare filled and buriedbyeither coarse-grained ma­rine clasts (eg,Punta Higiiero) or a semi-lithified sandyter­restrial red clay (eg, East Island). Hostrocks contain a highpercentage of siliciclastic material and have high primaryporosity. Mostpipesarebounded bya haloofinduration whichextends up toa few centimeters laterally anda couple ofdeci­meters below the open pipe.The pipeswhich are filledwithred clayalsohavea liningof palebrown, fine-grained, lami­natedcalcrete a couple ofmillimeters thick. In places thehaloof induratedrockis slightly emergent as theexhumation pro­cessdenudes the overburden and surrounding rocksurface.

The pipes formed along focused vadose flow paths

bydissolution ofthecarbonatecement (isopachous microspar)and carbonate clasts in the host rock, and re-deposition ofcalcite in the form of a thin calcrete margin. Stemflow is thelikely mechanism for focusing of flow but there is no clearevidence for this in the form of vegemorphs. In the East Is­landsite,siliciclastics dissolved outof thehostrockremainedin thepipe,settledsomewhatandwereaugmentedbyredclayswhich nowform thepipefill. Thiscouldindicate pipeforma­tion eitherduring or after red claydevelopment. The PuntaHigiiero siteshows a more complex history withno evidenceofa clayfilling. Mostof theoriginal, possibly sandy, fill wasremoved prior to thepipesbeingrefilled withcoarse-grainedmarinesediment. The pipedrockunitwas thenplanedoffatthetopandcoveredwithcoarse-grainedmarinedeposits whichhave in tum been modified by later pipe formation and fill­ing. Both stages of pipingmayhaveoccurred within isotopesubstage 5e.

ACKNOWLEDGEMENTS

This work was supported mainlyby USGS, PuertoRicoand in part by an NSERC Research grant to JL. Fieldassistance was provided byRam6n CaraScuillo. Manythanksto1. Carew for helpful comments in the manuscript.

REFERENCES

ADAMS, RA., 1983, General Guideto the Geological Fea­tures of San Salvador, inGerace, D.T.(ed)FieldGuideto the Geology of San Salvador, Bahamian Field Sta­tion, San Salvador, Bahamas, p. 1-66.

AHARON, P. and CHAPPELL, 1., 1986, Oxygen isotopes,sealevelchanges and the temperature history ofa coralreefenvironment in NewGuinea overthelast lOS years:Palaeogeography, Palaeoclimatology, Palaeoecology,v. 56, p. 337 379.

BAKER, G., 1943, Features of a Victorian limestone coast­line: Journal ofGeology, v. 51,p. 359-386.

BAKER, G., 1958, Stripped zonesat cliffedges alonga highwave energy coast, Port Campbell, Victoria: Proceed­ingsoftheRoyalSociety ofVictoria, v.70,p. 175-179.

BAUGHEN, DJ. and WALSH, P.T., 1980, Paleokarst Phe­nomena in the Carboniferous Limestone of Anglesey,NorthWales: Transactions oftheBritish Cave ReserchAssociation, v. 7, p. 13-30

BIRD,E.C.F., 1970, Shorepotholes atDiamond Bay, Victoria:VictoriaNaturalist, v. 87,p. 312-318.

BRElZ, J.H., 1960, Bermuda: a Partially drowned, LateMature, Pleistocene Karst: Bulletin of the GeologicalSociety ofAmerica, v. 71, p. 1729-1754.

BRINK, A.BA and PARlRIDGE, T.e., 1980, The natureandgenesis ofsolution cavities (Makondos) inTransvaalcavebreccias: Palaeontology africa, v. 23, p. 47-49.

BRUNSDEN, D., OOORNKAMP, J.C., GREEN, C.P. andJONES, D.K.e., 1976, Tertiary and Cretaceous sedi­mentsin solution pipes in the Devonian Limestone ofSouth Devon, England: Geological Magazine, v. 113,

181

DISSOLUTION PIPES IN NORfHERNPUERlO RICO

p.441-447.BURNABY, T.P., 1950, The tubular chalk stacks of

Sheringham: Proceedings of the Geological Associa­tion, v. 61, p. 226-241.

CAREW, JL., and MYLROIE, J.L., 1991,Some pitfalls inpaleosol interpretation in carbonate sequences: Carbon­atesandEvaporites, v. 6, p. 69-74.

CAREW, JL. and MYLROIE, J.E., 1994,Geology andKarstof San Salvador Island, Bahamas: A field trip guide­book. Bahamian Field Station, San Salvador Island,Bahamas,32 p.

CAREW, JL. and MYLROIE, J.E., 1995(in press),Deposi­tionalmodelsand stratigraphy for theQuaternary geol­ogy of theBahamaislands, inCurran,RA, andWhite,B., (eds), Terrestrial and shallow marinegeology oftheBahamasand Bermuda: Geological Society ofAmericaSpecial Paper, 300.

CHAPPELL, J. and SHACKLETON, NJ., 1986,Oxygen is0­topesand sea level: Nature, v.324,p. 137 140.

COElZEE, E, 1975,Solution Pipesin CoastalAeolianites ofZululand and Mozambique: Transactions of the Geo­logical Society of South Africa, v. 78, p. 323-333.

DAY, AE., 1928,Pipes in the CoastSandstone of Syria: TheGeological Magazine, v.65, p. 412-415.

DEBRUIJIN, RG.M., 1983,Someconsiderations on thefac­tors that influence the formation of solution pipes inchalk rock:BulletinoftheInternational Association ofEngineering Geology, v. 28, p. 141-146.

DUNHAM, RJ., 1962, Classification of carbonaterocksac­cordingtodepositional texture, in W.E. Ham(ed) Clas­sification ofCarbonateRocks: AmericanAssociation ofPetroleum Geologists Memoirs, v. 1,p. 108-121.

ESTEBAN, M. and KLAPPA, C.E, 1983, Subaerial expo­sure environments, in Scholle, P.A, Bebout, D.G, andMoore, C.H. (eds) Carbonate Depositional Environ­ments:American Association of Petroleum GeologistsMemoirs, v, 33, p. 2-54.

FAIRBRIDGE, R W., 1950, The geology and geomorphol­ogy of Point Peron, Western Australia: Journal of theRoyalSocietyofWestern Australia, v. 34, p. 35-72.

FALCON, NL., 1929,Correspondence: Pipes in coastsand­stone: TheGeological Magazine, v. 66, p. 48.

FLUEGEL, E., 1982, Microfacies Analysis of LimestonesSpringer-Verlag, Heidelberg.

FOLK,RL., 1962, Spectral subdivision of limestone types,in W.E. Ham (ed) Classification of Carbonate Rocks:American Association of Petroleum Geologists Mem­oirs, v. 1, p. 62-84.

FOOS, A., 1987, Paleoclimatic interpretations of paleosolson San SalvadorIsland, Bahamas, in RACurran (ed)Proceedings of the 3rd. symposium on the Geology ofthe Bahamas,San SalvadorIsland,bahamianfieldSta­tion,p. 67-72.

FORD, D.F., 1984, Paleokarsts in Britain: Cave Science,Transactions of the British CaveResearch association,v. 11,p. 246-264

FORD, D.C. and WILLIAMS, P.W., 1989, Karst Geomor-

phologyand Hydrology. Unwin Hyman, London, 601p.

HARRISON, RS. andSTEINEN, RE, 1978, Subaerialcrusts,caliche profiles, and breccia horizons: Comparison ofsome Holocene and Mississippian exposure surfaces,Barbados andKentucky: Geological Society ofAmericaBulletin, v. 89, p. 385-396.

HEARIY,P.J. and KINDLER, P., 1995,Sea-level HighstandChronologyfromStable CarbonatePlatforms (Bermudaand The Bahamas): Journal ofCoastal Research, v. 11,p.675-698.

HEARIY,P.J., VACHER, HL.,andMITTERER, RM., 1992Aminostratigraphy and ages of Pleistocene limestonesofBermuda: Geological Society ofAmericaBulletin, v.104,p. 471480.

HERWITZ, SR, 1993, Stemflow influences on the forma­tion of solution pipes in Bermuda eolianite: Geomor­phology, v.6. p. 253-271.

JAMES, N.P., 1972, HoloceneandPleistocene calcareouscrust(caliche) profiles: Criteriaforsubaerial exposure: Jour­nalofSedimentary Petrology, v. 42, p. 817-836

JENNINGS, IN., 1968, Syngenetic Karst in Australia, inWilliams, P.W. and Jennings,J.N. Contributions to thestudyofkarst,Research SchoolofPacificStudies, Aus­tralian National University, Department of GeographyPublication G/5,p. 41-110

KAYE,CA, 1959, ShorelineFeatures andQuaternaryShore­line Changes, PuertoRico: Geological Survey Profes­sional Papers, v. 317-B.

KlRKALDY, lE, 1950,Solutionof the chalk in the MimmsValley, Herts: Proceedings of the Geological Associa­tion, v. 61,p. 219-224.

KLAPPA, C.F., 1980,Rhizoliths in terrestrial carbonates: clas­sification, recognition, genesis and significance: Sedi­mentology, v. 27. p. 613-629.

lEFEBRVE, G.,LEGRAND, R, and MORTELMANS, G.,1968,Essaim depuitsnaturelsa Kain: Societe Belgiquede Geologie, Paleontologie, etHydrologie, Bulletin, v.76, p. 63-66.

LINK, A.G., 1967,LatePleistocene-Holocene climaticFluc­tuations; Possible solution pipe-Foibarelationships; andtheevolution oflimestonecavemorphology: ZeitschriftfUr Geomorphologie, v. 11,p. 117-145.

UVINGSTON, W., 1944, Observations on the structure ofBermuda: Geographical Journal, v.104, p. 40-48.

MOORE, W.S., 1982, Late Pleistocene sea-level history, inIvanovich, M and Harmon,RS. (eds.)UraniumSeriesDisequilibrium: Application toEnvironmentalProblems.Clarendon Press,Oxford, 571 p.

MOUNT, J., 1985, Mixed siliciclastic and carbonate sedi­ments: a proposed first-ordertextural andcompositionalclassification: Sedimentology, v.32, p. 435-442.

MYLROIE, lE., Ed., 1988,FieldGuideto theKarstGeologyof San Salvador Island, Bahamas. Port Charlotte,Florida,Bahamian Field Station, 108p.

MYLROIE, J.E., and CAREW, JL., 1988,Solutionconduitsas indicators ofLateQuaternary sealevelposition: Qua-

182

LUNDBERG ANDTAGGARf

ternary Science Reviews, v. 7, p. 55-64.MYLROIE, J.E., CAREW~ J.L., SEALEY, N.E. and

MYLROIE, lR., 1991, Cave development on NewProvidence Island and Long Island, Bahamas: CaveScience, v. 18,p, 139-151.

PLAS,L. VAN DER,TOBI,AC., 1965,A chart forjudgingthereliability ofpointcountingresults: AmericanJour­nalofScience, v.263, p. 87-90.

PRICE,D.G. and VERHOEF, P.N.W., 1988,The stabilityofabandoned mine workings in the Maastrichtian lime­stones of Limburg, The Netherlands, in Bell, EG.,Culshaw, M.G.,Cripps,J.C.and Lovell, M.A Proceed­ings of tje 23rd annual conference of the EngineeringGroup of the Geological Society, Teesside Polytech.,Middleborough, UK: Engineering Geology SpecialPublications, v. 23, p. 193-204.

PRASAD, G., 1983, On theoriginofredclayson Quaternarylimestone along the East Africancoast: Zeitschrift furGeomorphologie, NF. Supplement, v.48, p. 51-64.

PROUTY, J.S. and LOVEJOY, D.W., 1992, Remarkable cy­lindricalsolution pipesin Coquinasouthof Baffin Bay,Texas: Transactions of the Gulf Coast Association ofGeological Societies, v. 42, p. 599-606.

RADTKE,u,andGRUN,R., 1990, Revised ReconstructionofMiddleand LatePleistocene sea-level changesbasedonnewchronologie and morphologic investigations inBarbados, WestIndies: Journal ofCoastal Research, v.6, p. 699-708.

RASMUSSEN, K.A and NEUMANN, AC.,1988, Holoceneoverprints of Pleistocene paleokarst: Bight of Abaco,Bahamas, in James, N.P. and Choquette, P.W. (eds)Paleokarst Springer-Verlag, NewYork. p. 132-148.

ROSE,P.R, 1970,Stratigraphic Interpretation of Submarineversus Subaerial Discontinuity Surfaces: an examplefrom the Cretaceous of Texas: Geological Society ofAmericaBulletin,v. 81, p. 2787-2798.

ROSSINSKY, V.l,andWANLESS,H.R, 1992, Topographicandvegetative controls oncalcrete formation, TurksandCaicos Islands, British West Indies: Journal of Sedi­mentary Petrology, v. 62, p. 84-98.

ROSSINSKY, V. Jr., WANLESS, RR. and SWARf, P.K.,1992, Penetrative calcretes and their stratigraphic im­plications: Geology, v.20, p. 331-334.

SHINN, E.A. and LIDZ, BR., 1988,Blackened LimestonePebbles: FireatSubaerial Unconformities, inJames, N.P.and Choquette, P.W. (eds) Paleokarst Springer-Verlag,NewYork. p. 117-131.

SMITH, B.J. and McALLISTER, rr, 1995, Mineralogy.chemistry and paleoenvironmental significance of an

Early Tertiary Terra Rossa from Northern Ireland: Apreliminary review: Geomorphology, v. 12,p. 63-73.

SPENCER, T., 1981, Micro-topographic change ofcalcarenites, GrandCaymanIsland,WestIndies: EarthSurface Processes and Landforms, v. 6, p. 85-94.

TAGGARf, B.E., 1992,Tectonic and eustaticcorrelations ofradiometrically dated marine terraces in northwesternPuertoRicoand Islade Mona.PuertoRico. Mayagiiez,University of PuertoRico, unpublished Ph.D. disserta­tion,252 p.

THOREZ, J., BULLOCK, P.,CAIT, lA., and WEIR. AR,1971, The petrography and origin of deposits fillingsolution pipes in the chalk near South Mimms,Hertfordshire: Geological Mageazine, v. 108, p. 413­423.

VACHER. RL., and HEARTY, P., 1989, History of Stage5SeaLevelin Bermuda: Review with newevidenceof abriefriseto presentsea levelduring Substage 5a: Qua­ternary Science Reviews, v. 8, p. 159 168.

WALKDEN, G. and DAVIES, J., 1983,Polyphase erosion ofsubaerial omission surfaces in the Late Dinantian ofAnglesey, North Wales: Sedimentology, v. 30, p. 861­878.

WEBB, G.E., 1994, Paleokarst, paleosol, and rocky-shoredeposits at the Mississippian-Pennsylvanianunconformity, northwestern Arkansas: Geological So­cietyofAmericaBulletin, v. 106,p. 634-648.

WEST, I.M.,1973,Carbonatecementation ofsomePleistocenetemperate marine sediments: Sedimentology, v. 20, p.229-249.

WILLIAMS, RS., Jr., 1965, Geomorphology of a portionofthenortherncoastalplain ofPuertoRico. PennsylvaniaStateUniversity, unpublished Ph.D.dissertation, 191p.

WILSON, RC.L., 1960,ThePliocenebenchandrelatedprob­lemsofEastKent:Journal oftheUniversity ofSheffieldGeological Society, v.3, p. 117-118.

WRIGHT, V.P., 1983, The polyphase karstification of theCarboniferous Limestone in SouthWales, in Sweeting,M.M.andPatterson, K.(eds)NewTrendsin KarstGe0­morphology, Geo-Abstracts, Norwich, p. 569-577.

WRIGHT, V.P., 1988,Paleokarsts and Paleosols as Indicatorsof Paleoclimate and Porosity Evolution: a Case studyfrom the Carboniferous of SouthWales, in James,N.P.and Choquette, P.W. (eds) PaleokarstSpringer-Verlag,NewYork, p. 329-341.

Received: October10,1995Accepted: October22, 1995

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