Trace Fossils in Cretaceous-Tertiary (KT) Boundary Beds in Northeastern Mexico: Implications for Sedimentation during the KT Boundary Event

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1 RESEARCH LETTERS 593 Trace Fossils in Cretaceous-Tertiary (KT) Boundary Beds in Northeastern Mexico: Implications for Sedimentation during the KT Boundary Event A. A. EKDALE Department of Geology and Geophysics, University of Utah, Room 719, 135 South 1460 East, Salt Lake City, UT WOLFGANG STINNESBECK Geologisches Institut, Karlsruhe Universität, Postfach 6980, D Karlsruhe, Germany PALAIOS, 1998, V. 13, p Cretaceous-Tertiary (KT) sequences in Nuevo León and Tamaulipas, Mexico, contain trace fossils that shed important light on the nature and duration of deposition of the KT boundary strata in that region. The KT clastic sequence in northeastern Mexico typically is divided into three distinct sedimentary units which represent distinctly different depositional events. Unit I is an unbioturbated, laminated deposit of alternating smectite grains and calcite spherules. Unit II is a sandstone that is mostly unbioturbated, but a few spherule-filled burrows occur near the base of the unit. Several burrows were truncated by overlying sand layers within Unit II, indicating that they were excavated following deposition of the first sand layers and then filled with spherules, scoured, and overlain by more Unit II sand. Unit III consists of alternating sandstone, siltstone, and shale that contain abundant trace fossils, including Chondrites, Ophiomorpha, Planolites, and Zoophycos. The nature of the trace fossil occurrences attest to at least three successive colonization episodes of the accreting substrate. The sandstone beds of Unit III were deposited episodically, and burrowing occurred during the period of deposition, not after deposition had ceased. The burrows were filled with late Cretaceous sediment. Trace fossil evidence indicates, therefore, that the entire KT clastic sequence must have been deposited over a long period of time. If the spherules in Unit I are material derived from an extraterrestrial impact, that impact must have predated the extinction of Cretaceous plankton by a significant time interval, which is represented by the periods of deposition of Units II and III. The ichnologic information indicates episodic deposition of Units II and III over an extended time period. Thus, the event that produced the calcite spherules in Unit I is not directly related to the Cretaceous plankton extinctions at the KT boundary, which occur at the top of Unit III. INTRODUCTION Geological and geophysical evidence from a large structure in the subsurface beneath the town of Chicxulub on the Yucatan Peninsula, Mexico, indicate that an extraterrestrial impact occurred on the shallow-marine shelf there at the end of the Cretaceous Period (Hildebrand et al., Copyright 1998, SEPM (Society for Sedimentary Geology) /98/ /$ ; Sharpton et al., 1992). On the other side of the Gulf of Mexico, sedimentary sequences containing the Cretaceous-Tertiary (KT) boundary in northeastern Mexico consist of clastic units that have been interpreted by some workers as deposits resulting from the effects of an impact in Yucatan (Smit et al., 1992, 1994a, 1994b, 1996; Bohor, 1994, 1996; Clifton and Dott, 1994). According to this scenario, the entire clastic sequence would have been deposited in a matter of hours to days as a consequence of the KT bolide impact, and the KT boundary would be placed at the base of the sequence (Smit, 1994b). In contrast, other workers have interpreted these clastic sediments as deposits unrelated to an impact on Yucatan and preceding the KT boundary event (Stinnesbeck et al., 1993, 1996; Keller et al., 1993a, 1994a, 1994b, 1997; López-Oliva and Keller, 1996; Stinnesbeck and Keller, 1996). In that scenario, the KT boundary, as defined biostratigraphically and by a small iridium anomaly, would be placed above the clastic sequence. Trace fossils and other ichnologic evidence in these KT sequences in northeastern Mexico shed important light on the nature and duration of deposition of the boundary strata. This paper describes the trace fossil associations in their sedimentologic context in KT beds at key sites in Nuevo León and Tamaulipas, Mexico. GEOLOGIC SETTING The Chicxulub structure, where a bolide impact apparently occurred at or near the end of the Mesozoic Era, is located on the northern edge of the Yucatan Peninsula. In latest Cretaceous time, this area was submerged on a vast, shallow-marine, carbonate platform; hence, the bolide must have landed in the sea. More than 1000 km northwest of Chicxulub is a northsouth band of outcrops in which the KT boundary occurs. These are situated east of the Sierra Madre Oriental mountain front in the states of Nuevo León and Tamaulipas, Mexico (Fig. 1A). These sections consist of uppermost Cretaceous (Upper Maastrichtian) fine-grained marls in the Mendez Formation, overlain by clastic beds that range from only 2 cm thick at La Parida to more than 7 m thick at El Peñon. This clastic sequence is overlain by lowermost Tertiary (Lower Paleocene) fine-grained marls of the Velasco Formation. It is the KT clastic beds that have attracted considerable attention because they record the

2 594 EKDALE & STINNESBECK geologic conditions and events associated with the era boundary. The outcrops studied for this paper include El Mimbral, El Mulato, El Peñon, Rancho Canales, La Lajilla, La Parida, La Sierrita, Los Ramones and several sites near Linares (Fig. 1B). They are located in a band that parallels the late Cretaceous paleoshoreline and extends today from Los Ramones, some 40 km east of Monterrey, southwards to El Mimbral, some 80 km southeast of Ciudad Victoria. The marine facies in these sections become shallower to the north, with those at Los Ramones being the shallowest. Still shallower facies occur to the west and northwest, represented by the coastal deltaic sediments of the Difunta Group (Murray et al., 1962; McBride et al., 1975; Wolleben, 1977). The KT clastic beds are divided into three distinct sedimentary units. Unit I is characterized by laminae of alternating smectite grains and calcite spherules and apparently is unbioturbated. Rare glass shards in Unit I may be of tektite origin (Smit et al., 1992, 1994a, 1994b; Koeberl, 1994) or volcanic origin (Stinnesbeck et al., 1993, 1994; Keller et al., 1994a, 1994b; Robin et al., 1994). Unit II is texturally homogeneous sandstone that is mostly unbioturbated, but a few spherule-filled burrows occur near the base of the unit. Unit III consists of alternating sandstone, siltstone, and shale that contain abundant trace fossils of several types. Detailed sedimentologic descriptions and stratigraphic sections of these outcrops have been published previously (Stinnesbeck et al., 1993, 1996; Keller et al., 1994c, 1997) and therefore are not repeated here. The upper part of the Mendez Formation and the KT clastic beds all are in the Upper Maastrichtian Abathomphalus mayaroensis foraminiferal zone. The faunal assemblages in this interval also contain Plummerita hantkeninoides, a short-ranging species that evolved 170,000 to 200,000 years prior to the KT boundary and disappeared at the boundary (Pardo et al., 1996). This species is present below and within all KT clastic deposits in northeastern Mexico, except for those at Mulato (López-Oliva and Keller, 1996). Rich and diverse latest Maastrichtian planktic foraminiferal assemblages also are present in a 5- to-10-cm-thick marl or marly limestone layer above the KT clastic deposits at Mulato, La Lajilla, and La Parida. The first Tertiary foraminiferal species occur above a thin clay layer that immediately overlies this marl layer of the Unit III sandstones (Keller et al., 1994a, 1994b, 1997; Keller and Stinnesbeck, 1996a; López-Oliva, 1996; López-Oliva and Keller, 1996). TRACE FOSSIL DISTRIBUTION IN THE KT BOUNDARY BEDS Unit I FIGURE 1 Location of study area (adapted from Keller et al., 1994a, figs. 1 3). (A) Regional map. Stippled pattern indicates the study area in northeastern Mexico, which is located more than 1000 km northwest of the Chicxulub impact site on the Yucatan Peninsula. (B) Map of the study area. Stars denote the specific study sites. Unit I, commonly referred to as the spherule layer, contains no trace fossils at any site. The sediments typically are evenly laminated, displaying alternating laminae of carbonate spherules and clay (Fig. 2). The spherules are rounded, spheroidal grains ranging in size from 1 to 5 mm, and most are filled with blocky calcite, sometimes accompanied by clay minerals (Keller et al., 1994b). They have been interpreted variously as altered microtektites derived from an impact (Smit et al., 1992) and as detrital

3 TRACE FOSSILS IN KT BOUNDARY BEDS IN MEXICO 595 FIGURE 2 Unbioturbated spherule layer in Unit I at El Mimbral, which contains alternating laminae of calcite spherules and clay. Large, light-colored patches are rounded, flattened, claystone clasts. Scale bar: 1 cm. grains derived from shelfal deposits occurring to the west (Stinnesbeck et al., 1993). Scour features are evident at the base of Unit I in some localities, especially El Mimbral. Pre-modern (Holocene?) calcareous rhizocretions occur sparsely in the upper part of Unit I at El Mimbral. These are elongate, thick-walled, white structures that are either hollow or contain brown, carbonaceous material. Because their occurrence is restricted to sites near modern cracks in the rock, these structures are interpreted as concretionary features around Recent plant roots that have nothing at all to do with the deposition of Unit I sediment. Unit II FIGURE 3 Siliceous sandstone in Unit II at El Mimbral. (A) Texturally and compositionally homogeneous sandstone in the middle part of the unit. Scale bar: 2 cm. (B) Laminated sandstone, including some spherule-rich laminae, in the lower part of the unit. Scale bar: 1 cm. Unit II beds are texturally and compositionally homogeneous for the most part. Most of the strata in Unit II typically show little or no evidence of primary sedimentary structures (Fig. 3A). Plant debris (wood and leaf fragments) occur in layers at the base of the unit. In places, the basal part of the unit is very well-laminated with alternating fine-grained and coarse-grained sand in planar laminae (Fig. 3B). Above this is very well-sorted, very finegrained, unlaminated, well-indurated sandstone. A few spherule-filled burrows occur in the lower part of Unit II at El Peñon (Fig. 4; see also Keller et al., 1997, fig. 13B). These are poorly formed burrows; that is, they are irregular in shape and non-uniform in diameter with an unordered, coarse-grained fill and an indistinct margin. They are subvertical, gently curved to J -shaped, unlined burrows with diameters varying from 5 to 15 mm. The burrows are recognized by their coarse fill, including unbroken spherules in some cases, which differs markedly from the fine-grained host sediment. These burrows were truncated by overlying sand layers within Unit II, indicating that they were excavated as open burrows following deposition of the first sand layers and then filled with spherules, scoured, and overlain by more Unit II sand. Thus, it appears that these burrows were emplaced during, not following, the Unit II sedimentation event. Unit II is a curious layer; it contains mud clasts, spherule-filled burrows, and planar laminae in the lowermost few centimeters, but it is homogeneous, structureless and monotonous in the remainder. How could open burrows be emplaced during the early stages of deposition of this massive sandstone unit? There must have been some hiatus in sedimentation, even if short-lived, to allow burrowers of unknown biological affinities to colonize the sand. The burrows in Unit II probably were simple, shallow, open burrows, which could have been excavated very quickly. Unit III Unit III sandstone beds contain abundant and diverse trace fossils, consisting mostly of well-developed depositfeeding burrows (e.g., Chondrites, Ophiomorpha, Planolites, and Zoophycos) that represent permanent or semi-permanent colonization of the sediment (Figs. 5 to 9). It is a

4 596 EKDALE & STINNESBECK FIGURE 4 Spherule-filled burrow in homogeneous sandstone near the base of Unit II at El Peñon. (A) Close-up photograph of irregular, J -shaped burrow, in which dark-colored, unbroken, calcite spherules are evident. Scale bar: 1 cm. (B) Outline of burrow in the same photograph (same scale). tiered ichnocoenosis; that is, the sandy substrate was vertically partitioned to produce a composite ichnofabric. Planolites and other simple trails occupied a shallow tier within the substrate, and Zoophycos, Ophiomorpha, and Chondrites occupied progressively deeper tiers. Such trace fossils reflect occupation of the sediment by predominantly deposit-feeding infauna for an extended period of time. TRACE FOSSIL DESCRIPTIONS Chondrites Chondrites is a highly branched feeding burrow produced relatively deeply in the sediment by an unidentified organism. Chondrites isp. is abundant in Unit III sandstones, commonly in direct association with Ophiomorpha and occasionally in direct association with Zoophycos. The typical Chondrites specimens exhibit very regular branching of straight tunnels at acute angles in mostly a horizontal plane in the fine sandstone (Fig. 5). Tunnel diameters range from less than 0.5 mm to about 1.0 mm. Some short segments of the vertical Chondrites shafts can be seen in vertical cross-section in a few beds at El Peñon (Fig. 6; see also Keller et al., 1997, figs. 13C and D). Ophiomorpha FIGURE 5 Chondrites exposed on bedding planes of fine sandstone within Unit III. These are deep deposit-feeding burrows consisting mainly of a highly branched system of dominantly horizontal tunnels. (A) Chondrites at El Peñon. Scale bar: 1 cm. (B) Chondrites at El Mimbral. Scale bar: 2 cm. Ophiomorpha is a pellet-walled burrow created as a fodinichnial structure by deposit-feeding crustaceans. Ophiomorpha nodosa is prominent and extensive in the KT boundary beds in Mexico (Fig. 7). The characteristic Ophiomorpha wall pellets are clearly evident, although they commonly are preserved as molds (cavities) where they have been plucked out by weathering. The Ophiomorpha tunnels typically are about 1.0 cm in diameter, and they are straight or gently curved in most cases. Both Y junctions ( 120 degree angles) and T junctions ( 90 degree angles) occur in approximately equal numbers, but the tunnels extend for long stretches (up to several tens of centimeters) between junctions. The observations of Ophiomorpha tunnel length and orientation suggest that the sandy substrate was sufficiently stable for long-term occupation by the crustacean burrowers that produced the Ophiomorpha. Most of the Ophiomorpha networks are horizontal. However, vertical Ophiomorpha shafts can be seen at several localities. One 10-cm-long vertical shaft was observed in Unit III at El Peñon, and several short, vertical shafts were observed near the base of Unit III at Rancho Canales. Subvertical Ophiomorpha shafts occur throughout the KT sandstone beds at Los Ramones. At all three localities, the shafts were truncated by overlying sandstone layers, demonstrating repeated episodes of erosion and burrowing.

5 TRACE FOSSILS IN KT BOUNDARY BEDS IN MEXICO 597 FIGURE 6 Vertical shaft of Chondrites in sandstone in Unit III at El Peñon. Note that the burrow shaft (white vertical burrow on lefthand side of photograph) has been truncated by scour and subsequently overlain by ripple cross-laminated sand. This clearly demonstrates that the Chondrites was emplaced during (not following) the deposition of Unit III. Scale bar: 1 cm. Ophiomorpha occurs on tops of at least two sand beds in the upper part of Unit III, as well as on the top of the uppermost Unit III bed at El Peñon. These beds are characterized by unidirectional, current-oriented, ripple crosslamination. The occurrence of Ophiomorpha in ripple cross-laminated sediment is not surprising, because Ophiomorpha is common in high-energy, clean sand deposits throughout the geologic record (Ekdale, 1988). Planolites Planolites is a simple, unlined, unbranched, horizontal deposit-feeding trail, which was produced within the sediment and filled with material that differs slightly from the host sediment. Small Planolites beverleyensis (a few mm in diameter) occur sparsely in the KT boundary beds. Because they were produced in a shallow tier of the sediment, they usually were obliterated by subsequent burrowing represented by the deeper tier trace fossils. Nevertheless, their presence is important in that they represent the shallow-burrowing members of the infaunal community. Zoophycos Zoophycos is a broadly arcuate, spreiten burrow created by an unidentified deposit-feeding worm. Zoophycos isp. commonly occurs alongside other trace fossils, especially Ophiomorpha, in Unit III (Fig. 8). Most of the Zoophycos specimens exhibit simple, horizontal spreiten without any minor lamellae between the major lamellae of the spreite. In most cases, the spreite has a rounded, non-lobate outline, but some lobate forms were seen. No pellets were observed within the spreite. The Mexican Zoophycos closely resemble the typical Zoophycos reported in Upper Cretaceous (Coniacian) limestones in west-central Kansas (Frey and Howard, 1970, 1982) and in Upper Cretaceous (Campanian) sandstones in the Book Cliffs and Wasatch Plateau of central Utah (A. A. Ekdale, personal observations). Although a few Zoophycos specimens with a lobate outline were observed in the FIGURE 7 Ophiomorpha nodosa burrow networks in sandstone in Unit III. The burrow-fill sediment is preserved, but the characteristic wall pellets have been removed by weathering and erosion, leaving behind external molds of the pellets (indicated by arrows). (A) Ophiomorpha at El Peñon. Scale bar: 2 cm. (B) Ophiomorpha at Rancho Canales. Scale bar: 2 cm. El Peñon sections, these do not closely resemble the highly lobate, antler-shaped, pellet-lined Zoophycos that typify the uppermost Cretaceous (Upper Maastrichtian) strata in the KT sections along the Bay of Biscay coast of southern France and northern Spain (A. A. Ekdale, personal observations). The Mexican Zoophycos specimens also differ from the multi-whorl, pelleted Zoophycos in the uppermost Cretaceous (Upper Maastrichtian) Danish chalks described by Ekdale and Bromley (1991) or in the Upper Cretaceous-Tertiary Amuri Limestone on the South Island of New Zealand described by Ekdale and Lewis (1991). Thus,

6 598 EKDALE & STINNESBECK FIGURE 9 Schematic diagram of idealized tiering relationships of the common trace fossils in the ichnocoenosis of Unit III in the KT boundary beds. In order of increasing tier depth, they are Planolites, Zoophycos, Ophiomorpha, and Chondrites. The Planolites apparently was emplaced at a much shallower depth in the sediment than the other three ichnogenera. The deepest tier (Chondrites) extended at least 14 cm below the water-sediment interface. SEDIMENTOLOGIC IMPLICATIONS FIGURE 8 Zoophycos exposed on sandstone bedding planes within Unit III. These are highly organized deposit-feeding burrows that exhibit a spreite structure of densely packed, reworked sediment. (A) Slightly lobate Zoophycos at El Peñon. Scale bar: 3 cm. (B) Nonlobate Zoophycos at La Lajilla. Scale bar: 2 cm. the KT Zoophycos in northeastern Mexico are most similar to Upper Cretaceous Zoophycos that can be found in Cretaceous Western Interior Seaway deposits farther to the north in the western United States. TIERING RELATIONSHIPS OF TRACE FOSSILS The trace fossils of Unit III occur in a tiered ichnocoenosis, which represents habitat partitioning episodically and that burrowing of the sediment occurred during the period of deposition, not after deposition of the whole unit had ceased (Fig. 9). Ophiomorpha is seen most commonly in the geologic record as a shallow marine or marginal marine trace fossil, but it occurs in an offshore and presumably deep marine environment in the Mexican KT beds at Los Ramones, Rancho Canales, El Peñon, and elsewhere in the region. A pertinent question in this regard is why do we find rippled sand beds containing Ophiomorpha in this part of the geologic column (i.e., sandwiched between the apparently deep-water, hemipelagic Maastrichtian Mendez Formation and the hemipelagic Paleocene Velasco Formation)? Several hypotheses have been proposed. Smit et al. (1992, 1994a, 1996) suggested that these sand beds represent an unusual depositional event related to an impact tsunami, which would have piled up lots of clastic sediment stripped from nearshore environments and carried far out onto the shelf. However, these beds are not the product of a single depositional event. Hence, multiple tsunamis would be required to explain the observed sequence of strata. Alternatively, Bohor (1994) suggested that these sand beds comprise a single turbidite, which resulted from fluidized flow of sediment across an over-steepened slope, triggered by an impact tsunami or seismic event. However, the sequence of bed forms does not conform to typical turbidite facies, and the trace fossil evidence points towards multiple depositional events, including perhaps multiple tsunami or earthquake events, during an extended time period of several months, years, or possibly even centuries. A third suggestion is that a temporary lowering of sea level allowed transport of shallower water sediment via incised valleys and submarine canyons out into the basin (Stinnesbeck et al., 1993, 1996; Keller et al., 1994a,

7 TRACE FOSSILS IN KT BOUNDARY BEDS IN MEXICO 599 FIGURE 10 Ophiomorpha nodosa exposed on sandstone bedding plane within KT beds at Los Ramones. The meniscate structure of the burrow-fill material (indicated by an arrow) demonstrates that the burrowers actively backfilled their burrows with reworked sediment. Scale bar: 3 cm. FIGURE 11 Chondrites exposed on sandstone bedding plane within KT beds at Los Ramones. As at El Peñon and El Mimbral, these Chondrites burrows consist mainly of a highly branched system of dominantly horizontal deposit-feeding tunnels. Scale bar: 2 cm. 1994b). The depositional agents could have been gravity flows (e.g., debris flows and/or turbidity currents), triggered by the change in paleobathymetry. A well-documented eustatic lowering of sea level began several million years before the end of the Cretaceous and had an adverse effect on marine benthic invertebrate communities in this region during the late Maastrichtian. The sea-level drop was one of several major factors contributing to the collapse of Caribbean reef ecosystems and the extinction of rudistid bivalves, which occurred at least 1.5 million years before the KT boundary (Johnson and Kauffman, 1996). Normally, it would be intuitive that a major sea-level fall of hundreds of meters should be a long-term, if not permanent, situation. A short-term sea-level fall of as much as 80 to 100 m in the Maastrichtian has been suggested by several workers (Keller and Stinnesbeck, 1996a, 1996b; Pardo et al., 1996) based on benthic foraminiferal associations in various sites around the world. In fact, some evidence even has been cited for continental glaciation during the middle Maastrichtian in southern high latitudes (Barrera and Huber, 1990; Barrera, 1994). A cooling trend apparently continued during the late Maastrichtian and reached a maximum during the latest Maastrichtian sealevel lowstand, which probably coincided with the base of the Plummerita hantkeninoides Zone, about 200,000 years before the end of the Cretaceous (Barrera and Huber, 1990; Barrera and Keller, 1990; Barrera, 1994). During the last 100,000 years or more of the Maastrichtian and across the KT boundary, sea level apparently was rising (Schmitz et al., 1992; Keller et al., 1993b; Keller and Stinnesbeck, 1996a, 1996b). This may have been accompanied by greenhouse warming, perhaps caused by increased volcanic activity at the end of the Cretaceous. Farther to the north of the Mimbral area, thick sandstone beds at Los Ramones near the city of Monterrey contain extensive Ophiomorpha nodosa burrow networks (Fig. 10; see also Keller et al., 1997, fig. 13E) and well-developed Chondrites burrow systems (Fig. 11) at several levels. Lopez-Oliva (1996) confirmed a KT age for these deposits based on the presence of Plummerita hantkeninoides, which has a range that is restricted to the last 200,000 years of the Maastrichtian (see also Pardo et al., 1996). Although no spherule layer is present, and although Unit II and Unit III cannot be differentiated in the massive sandstone beds at this locality, these beds are nevertheless equivalent to the other KT clastic beds in northeastern Mexico. There are many distinct layers of detrital mud clasts within the sandstones, suggesting repeated event deposition. Similarly, numerous horizons of Ophiomorpha, located as much as 1.5 m below the top of the KT sequence, and at least one horizon of Rhizocorallium, located 1.5 m below the top of the KT sequence, represent repeated colonization events of the sediment by burrowers. The trace fossil evidence summarized in this paper demonstrates that the KT clastic beds in northeastern Mexico were deposited episodically and that burrowing of the sediment occurred during (not after) the period of deposition of Unit II and Unit III. Because of major differences in sediment composition, texture, and primary sedimentary structures (Keller et al., 1994a, 1994c; Stinnesbeck et al., 1996), as well as obvious breaks between them, it is apparent that Units I, II, and III represent distinctly different depositional events. Also, hiatuses of unknown duration separated some of the burrowing events within Units II and III. Thus, the entire KT clastic sequence must have been deposited over a fairly long period of time, certainly exceeding a few weeks or months, and probably exceeding a few years or decades. The very same trace fossils that can be seen in the KT beds also occur in similar associations in several promi-

8 600 EKDALE & STINNESBECK nent, planar-laminated to rippled sand beds in the upper part of the Maastrichtian Mendez Formation, which directly underlies the KT clastic beds. Marine sandstone beds occurring within the Mendez hemipelagic marl sequences were examined at several localities near the city of Linares, including outcrops near the main cemetery and the city dump. Zoophycos and small Chondrites accompany abundant Rhizocorallium and Scolicia in sand beds at the cemetery (Fig. 12A). Zoophycos and large Chondrites accompany abundant Rhizocorallium, Phycosiphon, and Helminthoida in sand beds at the city dump (Fig. 12B, C). It is clear, therefore, that Upper Cretaceous trace fossiliferous sand units containing Ophiomorpha, Zoophycos, Chondrites, and other associated ichnotaxa are not unique to the KT interval of this region. Farther to the northeast, marine shelf sequences at the KT boundary in central Alabama contain abundant trace fossils, including the same common ichnogenera that are found in the KT clastic units in northeastern Mexico (Savrda, 1993). In central Alabama, Chondrites, Planolites, Thalassinoides, and Zoophycos occur in uppermost Maastrichtian marl beds, and Planolites, Thalassinoides, and Ophiomorpha occur in lowermost Danian sandstone. Just as in the Mexican KT beds, trace fossil preservation and composite ichnofabrics in the central Alabama KT beds show obvious signs of multiple, repeated intervals of erosion and burrowing. The evidence strongly suggests that the KT sequences in central Alabama were deposited episodically as a result of sea-level dynamics. There is no convincing evidence that they represent deposits of a unique catastrophic event, such as a tsunami (Savrda, 1993). Some high-energy sand beds near the KT boundary in central Texas have been interpreted variously as deposits of a tsunami (Bourgeois et al., 1988) or a series of major storm events (Yancey, 1996). Although Yancey (1996) reported the occurrence of Thalassinoides there, no details of the ichnologic features in these beds have been described. CONCLUSION FIGURE 12 Common trace fossils in Upper Maastrichtian sandstone in the Mendez Formation near the city of Linares, Nuevo León. These trace fossils occur stratigraphically below the KT boundary beds and, therefore, demonstrate that burrowed sand beds are not restricted to the KT boundary in this area of Mexico. (A) Rhizocorallium at the city cemetery locality. Scale bar: 2 cm. (B) Zoophycos at the city dump locality. Scale bar: 3 cm. (C) Chondrites at the city dump locality. Scale bar: 2 cm. The KT clastic beds in northeastern Mexico shed interesting light on the impact scenario of the KT boundary. If the spherules in Unit I are impact-derived material from Chicxulub, then the Chicxulub impact predated the KT boundary and the appearance of Tertiary plankton by a significant time interval, probably on the order of several years at least, which is represented by the periods of deposition of Units II and III. The ichnologic information presented in this paper indicates episodic deposition of those beds over an extended period (i.e., years or decades rather than hours or days). Hence, the geologic event that produced the calcite spherules in Unit I is not directly related to the plankton extinctions that occurred at the KT boundary after deposition of Unit III. If one is to conclude that Unit I of the Mexican KT clastic beds contains airborne material (i.e., diagenetically altered spherules) derived from a bolide impact at Chicxulub, then that impact must have significantly pre-dated the KT boundary, as defined by planktic foraminiferal biostratigraphy, and the impact could not have been the sole immediate cause of the KT mass extinction event.

9 TRACE FOSSILS IN KT BOUNDARY BEDS IN MEXICO 601 ACKNOWLEDGMENTS The authors thank the University of Utah and Universidad Autónoma de Nuevo León for financial assistance in conducting this research. A grant to Wolfgang Stinnesbeck from the Deutsche Forschungsgemeinschaft (STI 128/2-1) is gratefully acknowledged. The authors also thank numerous colleagues, especially Gerta Keller, for cogent discussions regarding the KT boundary record in Mexico. 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