The Cretaceous-Tertiary boundary cocktail: Chicxulub impact triggers margin collapse and extensive sediment gravity flows

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The Cretaceous-Tertiary boundary cocktail: Chicxulub impact triggers margin collapse and extensive sediment gravity flows
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  INTRODUCTION The high-energy bolide impact at Chicxulubleft a distinct mark in Cretaceous-Tertiary (K-T)boundary sediments in the Gulf of Mexico andCaribbean region. For example, 900-m-thick impact breccia deposits are present 100 km fromthe crater (e.g., Sharpton et al., 1996). Tsunamideposits containing decimeter-size rip-up clastsare found along continental shelves in Texas andnorthern Mexico (e.g., Bourgeois etal., 1988).Gravity-flow deposits occur at the K-Tboundaryin Cuba (Iturralde-Vinent, 1992), Chiapas(Montanari et al., 1994), and Belize (O’Campoet al., 1996). Disturbed K-Tboundary units arealso reported in Haiti (Maurrasse and Sen, 1991)and in Deep Sea Drilling Project (DSDP) sites atthe base of the Campeche Escarpment (Alvarezetal., 1992).The age of proposed K-Tboundary depositson the shelf and in the basin of the Gulf of Mexico, however, has been disputed. Biostrati-graphic interpretations yielding Cretaceous agesfor spherule-bearing shelf sequences havecaused some (e.g., Keller et al., 1997) to ques-tion the association of these deposits, and thus of the Chicxulub event itself, with the profoundchanges in the Earth’s environment that occurredat the K-Tboundary (e.g., Hildebrand et al.,1991). In addition, Keller et al. (1993) concludedthat the K-Tboundary is unconformablethroughout the basinal Gulf of Mexico andCaribbean, whereas Alvarez et al. (1992) corre-lated coarse-grained basinal deposits to the K-Tboundary Chicxulub event.Here we describe a distinctive mixture of re-worked microfossils, impact-derived materials,and lithic fragments found in K-Tboundary stratain the basinal Gulf of Mexico and Caribbean.Lithologic and paleontologic evidence suggeststhat this mixture of particles was deposited bysediment gravity flows, probably triggered by theChicxulub impact. As a result of the pervasive re-working, biostratigraphy provides maximum agesfor the components that comprise K-Tboundarystrata, but not necessarily the age of deposition. METHODS AND RESULTS We have investigated the K-Tboundary inter-val at 11 Gulf of Mexico and Caribbean DSDPand Ocean Drilling Program (ODP) sites, andone land section (Beloc, Haiti) (Fig. 1). The sitesinclude K-Tboundary sections that appear to bestratigraphically complete as well as those thatcontain unconformities, and they range fromslope settings near the Yucatan to basinal loca-tions from the proximal Gulf of Mexico and distalCaribbean Sea (Fig. 2). Most of the sections drilledduring DSDPLegs 10 and 15 (Sites 86–152)were incompletely cored.Calcareous nannofossil biostratigraphy isbased upon high-resolution sampling (1–5 cm)across the K-Tboundary interval. 1 Nannofossilswere identified using a light microscope. Plankticand benthic foraminifers and other coarse silt- andsand-sized particles were observed in a few keysamples from Sites 536, 537, 538, and 540. Theseparticles were separated by washing the sedimentover a 44 µm sieve, then examined using binocu-lar and scanning electron microscopes (SEM).Mineralogies of key samples were determinedusing transmitted light microscopy and SEM-energy dispersive spectrometry.The srcinal nannofossil biostratigraphy ofSites86–152 was conducted before taxonomies andzonations were well established (e.g., Sissingh,1977); thus this study has resulted in significantrevisions (Fig. 2). Elsewhere the biostratigraphyis similar to published accounts (Watkins andBowdler, 1984; Sigurdsson et al., 1991).The K-Tboundary level is identified by thelowest occurrence of Paleocene microfossils;however, a suite of facies and considerable thick-nesses of sediments directly below this horizon inseveral sites (Fig. 2) may be related to the impactevent. Acombination of nannofossil biostra-tigraphy and diagnostic materials (includingspherules, shocked quartz, and Ir anomalies) isused to identify units associated with the impact Geology; April 1998; v. 26; no. 4; p. 331–334; 4 figures.331 The Cretaceous-Tertiary boundary cocktail: Chicxulub impacttriggers margin collapse and extensive sediment gravity flows Timothy J.Bralower*Charles K.Paull Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27599 R.Mark Leckie Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003 ABSTRACTAdistinctive mixture of reworked microfossils, impact-derived materials, and lithic frag-ments occurs in sediments at the Cretaceous-Tertiary boundary in the basinal Gulf of Mexico andCaribbean. We have named this mixture the Cretaceous-Tertiary boundary “cocktail.” Litho-logic and paleontologic evidence suggests that the cocktail was deposited by giant sediment grav-ity flows, apparently triggered by the collapse of continental margins around the Gulf of Mexicoas a result of the Chicxulub impact. As most microfossils in the gravity-flow units are reworked,biostratigraphy provides only maximum ages. Recognition of the cocktail is a reliable way toidentify Cretaceous-Tertiary boundary deposits in the basinal Gulf of Mexico and Caribbean. *E-mail: bralower@email.unc.edu. 1 GSAData Repository item 9838, biostratigraphicand sedimentologic data, is available on request fromDocuments Secretary, GSA, P.O. Box 9140, Boulder,CO 80301. E-mail: editing@geosociety.org. Figure 1.Paleogeographic recon-struction of Caribbean (after Pindelland Barrett,1990) showing location ofdepositional (black circles) and ero-sional (crosses) Cretaceous-Tertiaryboundary sequences investigated.Numbers refer to DSDP and ODPsites.Chicxulub impact site is indi-cated by large white circle. Data Repository item 9838 contains additional material related to this article.  and to characterize the K-Tboundary interval asbeing either erosional or depositional (Fig. 2).Spherule horizons at Sites 537 (Catoche Tongue),538 (Catoche Knoll), and 540 (base of the FloridaEscarpment) are described for the first time. MICROFOSSILREWORKINGAND ITSSTRATIGRAPHIC IMPLICATIONS Age-diagnostic nannofossils of multiple agesoccur in deposits directly below the K-Tbound-ary level at basinal Gulf of Mexico and Caribbeansites where there is no obvious unconformity(Fig.2). This mixture is indicative of reworking.Reworked assemblages contain markers that arediagnostic of the late Campanian to early Maas-trichtian (  Aspidolithus parcus subsp.  parcus, A. parcus subsp. constrictus ,  Eiffellithus eximius , Quadrum gothicum, Q. trifidum ,  Reinhardtitesanthophorus, R. levis ,  Lithastrinus grillii , and Tranolithus orionatus ), and Barremian to earliestAptian (  Nannoconus steinmannii ,  N. elongatus, N. minutus, and  Hayesites radiatus ).At Sites 536, 540, and 1001, and Beloc, olderreworked nannofossils are found mixed with lateMaastrichtian species (e.g.,  Micula murus and  Lithraphidites quadratus ). Rare specimens of   Micula prinsii were observed in uppermostMaastrichtian sediments from Site 536 (samples536-9-5, 130–131 cm and 536-9-6, 22–23 cm).The proportion of nannofossils that are diagnosticof Barremian to early Aptian age is estimated tobe <5%, and the proportion of age-diagnostic lateCampanian to early Maastrichtian species mayexceed 25%. Most taxa have long stratigraphicranges that extend through the Late Cretaceous.Species restricted to the late Maastrichtian are un-usually rare, indicating that most, if not all, nanno-fossils are reworked. Abundances of reworkednannofossil specimens decreases in the lower-most 5 cm of the Paleocene in all sections.The srcin of sediments near the K-Tboundaryat Sites 536 and 540 in the basinal Gulf of Mexico(Fig. 1) has been debated. Alvarez et al. (1992)proposed that these sediments were redeposited asa consequence of the Chicxulub impact based inpart on the distribution of spherules, shockedquartz, glass fragments, and an Ir peak that corre-sponds to the paleontological K-Tboundary(Fig.2). Keller et al. (1993) assigned these samesediments to an early or early late Maastrichtianage on the basis of the absence, or extreme rarity,of latest Maastrichtian planktic foraminifers, andthus questioned the relation of these deposits tothe K-Tboundary impact.In the sequence at Site 536, we found rare lateMaastrichtian planktic foraminifers (  Abathom- phalus mayaroensis [Fig. 3]  , Planoglobulinamulticamerata, Racemiguembelina powelli, R. fructicosa , and  Rugoglobigerina scotti ) mixedwith late Campanian to early Maastrichtian taxa.Nannofossil and planktic foraminiferal assem-blages are consistent with a depositional age of late Maastrichtian or younger, supporting theassociation of these units with the K-Tboundary(Alvarez et al., 1992).Extensive redeposition in Gulf of Mexico andCaribbean sections complicate the use of bio-stratigraphic data, which commonly only providemaximum ages, in establishing the sequence of events around the K-Tboundary. Clearly, otherapproaches are required. THE K-TBOUNDARYCOCKTAIL K-Tboundary strata from proximal basinalsites adjacent to the Campeche margin to themore distal central Caribbean contain a mixtureof reworked microfossils (nannofossils, andplanktic and benthic foraminifers), lithic frag-ments, and impact-derived materials. We termthe distinctive mixture of components the K-Tboundary “cocktail.” In some sites, cocktail unitsare separated from underlying Cretaceous andoverlying Paleocene strata by unconformities(Fig. 2). The relative abundance of componentsin the cocktail differs between sites as do thedurations of the hiatuses between cocktail unitsand Cretaceous and Paleocene strata.The sandy and pebbly chalk cocktail depositsat Sites 537 and 538 contain glass fragments(10–50µm diameter), quartz and sanidinegrains, granule-sized fragments of schist, gneiss,granite, and shallow-water limestone, fish teeth,echinoid spines, and reworked nannofossils andmid-Cretaceous neritic benthic foraminifers(Fig. 2). Many of the grains at the top of thecocktail units are coated by Fe-Mn oxides.Smectite spherules (largely hollow, spherical inshape, and 20–200µm diameter) are common atSite 537 (sample 537-3-2, 42 cm) (Fig. 3);smectite spherules are rarer at Site 538 (samples538A-21-1, 61 and 66 cm). Smectite spherulesare commonly formed by alteration of impact-derived glass tektites (e.g., Izett, 1991).Sandstone and chalk cocktail deposits at Sites536 and 540 (units 3 and 4 of Alvarez et al.[1992]) contain spherules, shocked quartz, andglass fragments (Alvarez et al., 1992). In addi-tion, we found fragments of shallow-water lime-stone and chalk, fish teeth, echinoid spines, andreworked nannofossils, planktic foraminifers,and mid-Cretaceous neritic benthic foraminifers(see also Sliter and Premoli Silva, 1984). Distalcocktail deposits from Beloc contain glass,shocked quartz, spherules (Sigurdsson et al.,1991; Maurrasse and Sen, 1991), and reworkednannofossils; distal deposits from Site 1001 con-tain shocked quartz, spherules, fragments of limestone and claystone (Sigurdsson et al., 1997),and reworked nannofossils (Fig. 2). 332GEOLOGY, April 1998 Figure 2.Stratigraphy and sedimentology of Cretaceous-Tertiary (K-T) boundary sections (seefootnote 1).Dashed lines indicate unconformities;solid lines indicate continuous deposition.Timing of K-T boundary sediment deposition assumes that spherule-bearing sediments areisochronous and that combined Site 536 and 540 sequence is complete.Reworked nanno-fossils:B = Barremian to early Aptian,Ce = Cenomanian,C = late Campanian to early Maas-trichtian,M= late Maastrichtian;F = reworked shallow-water benthic foraminifers (after Sliterand Premoli Silva,1984);Mn = Fe-Mn oxide coating;S = shallow-water carbonate fragments,L=limestone (undifferentiated) fragments,Met = metamorphic and igneous rock fragments;vertical black arrow = fining-upward sequence;horizontal arrow shows location of Ir anomaly;double black circles show locations of spherules;vertical scale bar by boundary depositsshows 5 m at Site 540,10 cm at other sites;Sl.= slumped;Ft.= faulted;W = winnowing.Age ofwinnowed cocktail deposits at Sites 538 and 540 are best estimates.Numbers shown alongSites 536 and 540 columns refer to Alvarez et al.(1992) lithologic units.Information on K-Tboundary characteristics was taken from Alvarez et al.(1992) for Sites 536 and 540;fromSigurdsson et al.(1997) for Site 1001;and from Maurrasse and Sen (1991) for Beloc.  The K-Tboundary cocktail is derived frommultiple sources so that microfossil biostra-tigraphy typically provides ages of its compo-nents, but not necessarily the timing of their finaldeposition. Latest Cretaceous marker species areexceptionally rare due to dilution by other cock-tail components and/or longer ranging taxa. Thus,the recognition of the cocktail itself provides areliable way of identifying K-Tboundary units atbasinal Gulf of Mexico and Caribbean sites. EVIDENCE FOR GRAVITYFLOWSDURINGTHE K-TBOUNDARYEVENT K-Tboundary sediments at the basinal Gulf of Mexico sites show lithologic evidence for depo-sition by sediment gravity flows. Sequences of coarse-grained deposits at Sites 536 and 540srcinally described by Alvarez et al. (1992) con-sist of poorly sorted pebbly mudstone containingchalk, mudstone, and bioclastic limestone clasts(unit 2) and cross-bedded sandstone containingangular chalk and bioclastic limestone clasts(unit 3) that grades up into chalk (unit 4) (Fig. 2).Alvarez et al. (1992) proposed that units 3 and 4were reworked by waves and currents triggeredby the Chicxulub impact.We interpret these unitsas turbidites because of their fining-upward grainsize and paleodepths well below wave base.The age of pebbly mudstone unit 2 at Site 540was srcinally interpreted as extending fromearly to late Cenomanian (Premoli Silva andMcNulty, 1984; Watkins and Bowdler, 1984).Our observations suggest that the entire unit islate Cenomanian or younger on the basis of rareoccurrences of the nannofossil  Lithraphiditesacutum in the matrix. The mudstone containsangular clasts of Albian chalk and mudstone andis mixed with decimeter-size clasts of Albianshallow-water limestone (Premoli Silva andMcNulty, 1984). Mudstone-supported clasts sug-gest redeposition by mud flows (e.g., Lowe,1982). Large clast size indicates that unit 2 wasderived from proximal strata, suggesting that nomicrofossils are of pelagic srcin. Even thoughthe age of the pebbly mudstone is not preciselyestablished, we tentatively associate this unitwith the K-Tboundary events on the basis of itsposition beneath other redeposited K-Tboundarystrata (Fig. 2).Fining-upward, sandy and pebbly chalk cock-tail deposits around the K-Tboundary at Sites 537(section 537-3-2, 11 to 45 cm), 538 (section538A-21-1, 57 to 75 cm), and 540 (sections540-30-2 and 540-30-CC, above the interval stud-ied by Alvarez et al. [1992]) are interpreted to beturbidite deposits. Microfossil biostratigraphyindicates that the depositional age of sediments atSite 537 is late Campanian or younger. In Sites538 and 540, reworked Cretaceous nannofossilsare mixed with early to late Paleocene nanno-fossils and planktic foraminifera. Abundance of Paleocene microfossils increases upward, sug-gesting extensive winnowing after deposition. RELATIONSHIPOFTHE K-TBOUNDARYCOCKTAILTO THECHICXULUB IMPACT The K-Tboundary cocktail contains a mixtureof impact-derived materials that may have settledthrough the water column, and redepositedmaterials laid down by gravity flows. Only thedeposits at Beloc and Sites 536, 540, and 1001have previously been attributed to the Chicxulubevent (e.g., Sigurdsson et al., 1991, 1997;Alvarez et al., 1992). However, identical re-worked nannofossil assemblages in most K-Tboundary deposits suggest a similar srcin. Thebiostratigraphy of K-Tboundary deposits atsome proximal locations (Sites 536, 537, units2–3 of Alvarez et al. [1992] at Site 540) providesmaximum ages ranging from Cenomanian tolatest Maastrichtian that are consistent with aK-Tboundary srcin, whereas in others (Sites538 and 540 [sections 540-30-2 and 540-30-CC])some impact-derived materials are winnowedinto upper Paleocene sediments (Fig. 2).Impact-generated gravity flows appear tohave caused erosion at sites throughout thebasinal Gulf of Mexico and Caribbean. Dura-tion of hiatuses (or coring gaps) in the sectionsstudied range from less than a nannofossilzone (Site 152) to about 60 m.y. (Site 537)(Fig. 2). In several sites (e.g., Sites 86, 94, 95,146, 151, and 152), sediment overlying the un-conformity is earliest Paleocene in age (nanno-fossil zones CP1 and CP2), suggesting thatpelagic sedimentation resumed shortly (mostly<~0.5 m.y.) after erosion of the missing UpperCretaceous section during the K-Tboundaryevent. At Sites 95, 146, and 152, reworked lateCampanian to early Maastrichtian and lateMaastrichtian nannofossil species are identi-fied in sediments 1–2 cm above the K-Tboundary unconformity (Fig. 2) but are absentin overlying Paleocene horizons. The same re-worked nannofossils found in K-Tboundarydeposits elsewhere indicates that the gravityflows reached large areas of the basinal Gulf of Mexico and the Caribbean Sea. SOURCE(S) AND EXTENTOFTHESEDIMENTGRAVITYFLOWS The K-Tboundary cocktail can be used totrace the srcin and path of gravity flows in thebasinal Gulf of Mexico and Caribbean.Severalcomponents in K-Tboundary deposits matchsediments found directly below the K-Tbound-ary unconformity on the Yucatan continentalmargin, suggesting that this location was a sourceof the gravity flows. The sedimentary rocksdirectly below the K-Tboundary unconformity atSites 95 (Campeche Escarpment) and 538(Catoche Knoll) are late Campanian to earlyMaastrichtian in age. These units are the sameage as angular chalk clasts in the sandstone (unit3) at Site 540 and a dominant component in thereworked nannofossil assemblage (Fig. 2). Sedi-mentary rocks directly below the K-Tboundaryunconformity at Site 537 (Catoche Tongue) areBarremian to early Aptian in age, which matchesreworked nannofossil assemblages found at otherdownslope sites. In Sites 537 and 538, K-Tboundary deposits also contain fragments of metamorphic rocks that were probably derivedby erosion of nearby basement. Rocks recoveredbeneath the unconformity at Sites 86 and 94(Campeche Escarpment) are mid-Cretaceousshallow-water carbonates, similar to fragmentsand redeposited neritic benthic foraminifersfound in K-Tboundary deposits at Sites 536(base of Campeche Escarpment), 537, 538, and540 (Sliter and Premoli Silva, 1984). The westernFlorida continental margin is another possiblegravity-flow source.The K-Tboundary gravity flows were aeriallyand vertically extensive. Reconstructions (Fig. 1)show the Caribbean sites to be as much as1000km from a potential gravity-flow source.The presence of the K-Tboundary cocktail atSites 537 and 538 located on Cretaceous topo-graphic highs (Schlager et al., 1984) illustratesthat the gravity flows engulfed a significant partof the lower water column. Given the distributionof redeposited sediments, and the proximity of possible sediment sources on continental margins GEOLOGY, April 1998333 Figure 3.A,B.Smectite spherules (scale bar = 10 µm):A:Sample 537-3-2,42–43 cm.B:Sample538A-21-1,66 cm.C,D. Abathomphalus mayaroensis Sample 540-31-1,26–27 cm (scale bar =100µm).C is edge view;D is umbilical view.  surrounding the Gulf of Mexico, Caribbean, andwestern Atlantic, the volume of K-Tboundarygravity-flow deposits may have been enormous. TRIGGER AND TIMINGOFK-TBOUNDARYSEDIMENTGRAVITYFLOWS The Chicxulub impact may have dramaticallyaltered sedimentation in the Gulf of Mexico. Thekinetic energy derived by the impact is estimatedat ~5 × 10 30 ergs, which is equivalent to 10 8 Mtof TNTor a Richter-magnitude 13 earthquake(Covey et al., 1994). Gravity data are consistentwith an oblique, south to north bolide trajectory(Schultz and D’Hondt, 1996). We postulate thatsufficient energy was transmitted to the Yucatanand surrounding continental margins to causemassive slope failure (Fig. 4). The collapse of continental margins around the Gulf of Mexicomay have generated the large tsunami waves thataffected shelf sedimentation (e.g., Bourgeoisetal., 1988).Submarine landslides probably were triggeredby the Chicxulub impact, and further erosionoccurred during the subsequent passage of high-energy gravity flows across the basinal Gulf of Mexico and Caribbean. Deposition of gravity-flow–borne material, which in cases includesspherules, through the Gulf of Mexico andCaribbean may have occurred in several phases,but much of this material would have accumu-lated within hours to days of the impact. At paleo-depths <2000 m, deposition of the entire K-Tboundary unitrequires several years at most, thetime it would take for clay-bound Ir to reach thebottom (e.g., Ledbetter and Sparks, 1979). CONCLUSIONS Cretaceous-Tertiary boundary sediments in thebasinal Gulf of Mexico and Caribbean are com-posed of variable proportions of reworked micro-fossils, lithic fragments, and impact-derivedmaterials. This distinctive mixture, termed theK-Tboundary cocktail, provides a reliable wayof recognizing boundary units. Lithologic andpaleontologic evidence suggests that the cocktailwas deposited by geologically instantaneousgravity flows generated by the collapse of sur-rounding continental margins, presumably as aresult of the Chicxulub impact. The gravity flowseroded large areas and acquired sedimentarycomponents from a variety of sources. ACKNOWLEDGMENTS We thank D. Bottjer, J. Bourgeois, R. Buffler, J.Pospichal, K. Stewart, and W. Sliter for reviews, and A.Hooper and R. Norris for discussions. We are gratefulto N. Smith and P. Weiss of the Ocean Drilling Programfor help with sampling. Research supported byJOI-USSAC grants to Bralower and Leckie. 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GEOLOGY, April 1998 Figure 4.Effect of Chicxulub impact onYucatan continental margin and proposedsrcin of the Cretaceous-Tertiary boundarycocktail and gravity flows.
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