JOHN A. CHAMBERLAIN, JR .(1), DENNIS O. TERRY, JR. (2), PHILIP W. STOFFER (3), MARTIN A. BECKER (4)
(1) Department of Geology, Brooklyn College, Brooklyn, NY 11210
(2) Department of Geology, Temple University, Philadelphia, PA 19122
(3) U. S. Geological Survey Library, Menlo Park, CA 94025
(4) Department of Physics & Geology, The College of New Jersey, Ewing, NJ 08628
AbstractWe have documented a distinctive zone of disrupted sediments over 300 km2 in the Badlands area of South Dakota. This Disrupted Zone (DZ) is located in the Maastrichtian Fox Hills Formation about 20 meters above its transitional contact with the Pierre Shale, and ranges from 0.5 to 5 meters in thickness. Based on Sr age dates, sedimentary features, and impact ejecta, we interpret the DZ as a distal manifestation of the end-Cretaceous Chicxulub Impact Event. The DZ occurs in an interval with uncommon, but significant fossil content. Scaphitid ammonites characteristic of the Jeletzkytes nebrascensis ammonite zone occur below and within the DZ, but not above it. In addition, the sections, including the DZ itself, contain nuculid and inoceramid bivalves, osteichthian and chondrichthian remains, crustacean crawling traces, leaf fragments, and disseminated, and sometimes carbonized, plant debris. Compared to earlier late Cretaceous faunas of the Western Interior, the Badlands K/T fauna is impoverished in that it contains relatively few species. This impoverishment is unrelated to the end-K impact, and is more likely the result of environmental conditions in the Seaway coupled with the high-stress conditions of local environments within which the Badlands fauna lived.
|Figure 1. Paleogeography of North America at the end of the Cretaceous Period based on recent ammonite biostratigraphy. WIS _ Western Interior Seaway. BNP _ Badlands National Park. Modified from Fig. 1 of Terry et al. (in review). Paleogeographic data from Kennedy et al. (1998).|
During late Cretaceous time the Western Interior Seaway, an epeiric sea of considerable extent, covered much of the mid-continent region of North America. Kennedy et al. (1998) suggest that during the Jeletzkytes nebrascensis zone of the Late Maastrichtian the Seaway extended northward from the Cretaceous Gulf of Mexico to North Dakota (Figure 1). However, the absence of marine sections that can be unequivocally dated to the latest Maastrichtian or to the Cretaceous/Tertiary transition (e.g. Obradovich, 1993) has generally been interpreted to mean that the Seaway had retreated completely from the northern plains region of the United States by the end of the Cretaceous. In this scenario, marine rocks of early Tertiary age, the Cannonball Formation for example, necessarily imply the temporary return of marine conditions during the early Tertiary.
The late Cretaceous succession exposed in and near Badlands National Park in southwestern South Dakota contains evidence that challenges this view. In two recent papers (Stoffer et al. 2001; Terry et al., in review), we describe a zone of distorted bedding in the marine rocks of the Badlands area (referred to here as the DZ) which resembles the highly contorted, chaotic bedding seen in some Gulf of Mexico K/T boundary sections (summarized in Smit et al. 1996). Moreover, the Badlands DZ interval contains impact ejecta and fossil occurrences which suggest association with biotic and geologic events that mark the end of the Cretaceous. The Badlands DZ is conformably overlain by a considerable thickness of what thus would be earliest Paleocene marine sediments. In suggesting continuous marine deposition across the K/T boundary in the Badlands area, this interpretation implies that the Seaway had not disappeared from the northern plains at the end of the Cretaceous, but was a significant paleogeographic feature in this area well into the Paleocene.
In Stoffer et al. (2001), we were primarily interested in describing the Late Cretaceous biostratigraphic framework of the Badlands, within which this K/T boundary interval occurs. In Terry et al. (in review) we focused on the physical and sedimentological evidence suggesting that this distinctive zone of disrupted bedding does indeed mark the K/T boundary. In the present paper we focus on the paleontologic character ofthis marine K/T boundary interval, and in what the fossils indicate regarding the paleoenvironmental conditions in the Badlands region at the end of the Cretaceous.
The Late Cretaceous stratigraphic record of the Western Interior Seaway consists of two marine units in the northern Great Plains: the Pierre Shale and Fox Hills Formation. The Pierre Shale is a fossiliferous, gray mudstone, locally calcareous, and is generally interpreted as the product of deposition in the distal, central part of the Seaway. The overlying Fox Hills Formation, also fossiliferous, is a regressive phase of sedimentation that tracked the retreat of the Western Interior Seaway at the end of the Cretaceous Period. It consists predominately of buff colored sands and silts, and is interpreted as the product of deposition in a littoral to sub-littoral near-shore setting. The biostratigraphy of these units is based heavily on the molluscan zonation for the upper Cretaceous established by earlier workers (Gill & Cobban, 1966; Waage, 1968; Speden, 1970; Kaufmann, 1977), and summarized in recent reviews by Kaufmann et al. (1993) and Cobban (1993).
In the Badlands National Park region, Tertiary pedogenesis has severely altered the underlying Cretaceous units (Retallack, 1983; Terry & Evans, 1994), making them difficult to recognize and degrading or destroying much of their original fossil content. As a result of these conditions, the Upper Cretaceous biostratigraphic framework for the northern Great Plains is based largely on exposures in such areas as the Missouri, Moreau, and Grand River valleys of central South Dakota, and the Old Woman anticline of eastern Wyoming, rather than on exposures in the Badlands region. However, our recent biostratigraphic work on Upper Cretaceous strata in Badlands National Park shows that the stratigraphy of these units in southwestern South Dakota correlates with better known areas to the east and west (Stoffer, 1998; Stoffer et al., 1998; Stoffer et al. 2001). In this work, we have established on the basis of stratigraphic and paleontologic evidence that although the Pierre Shale and Fox Hills Formation are considerably thinner in the Badlands than elsewhere, the normal sequence of Western Interior ammonite range zones is preserved.
GEOLOGY OF THE BADLANDS K/T BOUNDARY
During our initial investigations we discovered a zone of highly contorted bedding in the brightly colored rocks of the "Interior Zone" below the Tertiary White River Group (Stoffer et al. 1997). This disrupted zone (DZ) occurs in what we established on sedimentologic and paleontologic grounds as pedogenically altered, marine Fox Hills Formation (Stoffer et al. 1997; Stoffer 1998; Stoffer et al. 1998). Further investigation of the DZ indicates that it marks the K/T boundary in this region, and represents seismically induced slumping and liquefaction of unconsolidated nearshore sediments derived from the effects of shock waves emanating from the Chicxulub impact (Stoffer et al. 2001; Terry et al., in review).
The Badlands DZ is clearly an unusual feature as it is the only zone of distorted, convoluted bedding anywhere in the Upper Cretaceous beds of southwestern South Dakota (Stoffer et al., 1997). The DZ thus seems to mark the occurrence of some unusual late Cretaceous event. We argue from the following considerations, discussed in Stoffer et al. 2001, and Terry et al., in review, that this event is indeed the end-K Chicxulub impact:
DISRUPTED ZONE LOCALITIES :SEDIMENTOLOGY AND FOSSIL PRESERVATION
Our paleontologic reconstructions are based on three main outcrops of the DZ within the study area: Wilderness Access Trailhead (WATH), and Dillon Pass (DP), both in Badlands National Park, and Creighton (CR), about 50 km north of the Park (Figure 2). Each location has a unique combination of paleontological characteristics that provide insight into the nature of the fauna inhabiting the Western Interior Seaway during the K/T transition. Sage Creek Basin (SCB), also within Badlands National Park, provides additional information on DZ fossils. The stratigraphy and sedimentology of these sites is discussed more fully in Stoffer et al. (2001) and Terry et al. (in review).
|Figure 2. Map of the Badlands National Park (BNP) area showing locations of the K/T boundary sections discussed here. BNP in gray. CR - Creighton; DP - Dillon Pass; GTO - Grassy Tables Overlook; SCB _ Sage Creek Basin; and WATH - Wilderness Access Trailhead.|
The DZ exposure at Creighton, SD, is located in Section 27, T3N, R15E, on a gravel road about 10 km west of its intersection with Creighton Road. The exposure lies at the head of a gully just below the crest of the Cheyenne River Breaks. The Fox Hills Formation at Creighton is about 45 meters thick, and is composed of cross bedded sands and silts, ripple laminated silts, and herring bone cross stratification with an overall southeast flow direction. The DZ lies approximately 20 meters above the base of the gradational lower contact with the Pierre Shale. The DZ is 0.5 to 1 meter thick and is bounded above and below by undisturbed sediments. The main body of the DZ is composed of a singular, predominantly massive, but occasionally coherent, body of beige fine sand and silt (Figure 3). Internally, the sand preserves slump/roll structures and isolated 1 to 10 cm blocks of sediment with cross stratification.
Of the DZ sites so far discovered, Creighton has the best fossil preservation and the greatest biotic diversity. Invertebrate and vertebrate remains, as well as plant material, are found below, above, and within the DZ (Figure 3), but they become rarer with distance above the DZ, and, with exception of plant debris, all but disappear several meters above this horizon. Fossils are not clustered in concretions as is commonly the case among Cretaceous Western Interior Seaway units. Instead, fossils occur in small pockets of shell debris or as isolated, and often fragmentary skeletal elements. Original shell carbonate is present in some specimens, although such material is invariably chalky and friable rather than pristine. Many specimens are preserved as casts and molds, some of which are sharp and well defined. Plant fossils occur as lenses of amorphous, sometimes carbonized, debris with occasional larger leaf or twig fragments also present.
Figure 3. Fossil occurrences in K/T boundary interval at Creighton, SD. The K/T boundary lies at the top of the Disrupted Zone (DZ). White arrows show deformed sandy beds within the DZ. Vertical lines at right show stratigraphic ranges of common macrofossil groups at Creighton: S _ scaphitid ammonites; N _ nuculid bivalves; P _ leaf fragments and disseminated plant debris.
Figure 4: Fossil occurrences in the K/T boundary interval, Dillon Pass, BNP. The figure shows only the lower portion of the Disrupted Zone (DZ) and the beds immediately below it. White arrow shows deformed sandy beds within the DZ. FZ _ zone containing nuculid bivalves, rare scaphitid ammonites, and disseminated plant debris, with lenticular, unfossiliferous concretions at base.
The DZ at Dillon Pass is visible to the east of SD Rte240 as the highway descends from the Pinnacles to the base of Dillon Pass along the headwaters of Conata Creek. The outcrop discussed here is exposed in a steep arroyo cut by a tributary of Conata Creek on the west side of the highway in Section 20, T2S, R16E. At this locality the K/T boundary interval lies within the Fox Hills Formation, which at this site, as throughout Badlands National Park, comprises the lower part of the brightly colored, pedogenically altered Interior Zone. The base of the exposed section is marked by a massive pale yellow sandy silt that becomes glauconitic and then red in color at the top (Figure 4). The DZ overlies this siltstone, and is composed of 4.5 to 5 meters of massive to convoluted clayey sandstone and siltstone with occasional beds of resistant and convoluted sandstone (Figure 4). Directly above the DZ are undisturbed, unfossiliferous laminated mudstones, the first of which contains impact spherules. The first nonmarine sediments, the mudstones of the late Eocene Chamberlain Pass Formation, lie four meters above the top of the DZ.
Invertebrate, vertebrate, and plant fossils occur at Dillon Pass, but they are much less abundant than at Creighton. Most invertebrate fossils occur as isolated, and often fragmentary, molds and casts. None contain original shell material, although shell impressions sometimes show a thin, friable limonitic coating, often with traces of original shell ornamentation, which probably represents the remnants of geochemically altered and leached shell carbonate. Finely disseminated plant debris is also present. The fossils occur in the reddish to pale yellow siltstone directly below the DZ (Figure 4), and appear to be concentrated about 20-40 cm beneath the base of the DZ. They do not appear to extend below a thin band of lenticular concretions, visible in Figure 4, about 50 cm beneath the DZ. Unlike Creighton, no fossils have been found in the DZ itself. This situation may reflect leaching of original shell carbonate which occurred during the pedogenic alteration of the Dillon Pass section (Retallack, 1983; Terry and Evans, 1994). The geochemical effects of pedogenesis appear to diminish downward in the Dillon Pass section. Thus, organic remains high in the section, above and within the DZ, were probably obliterated, while those lower in the section, below the DZ, were degraded but not totally destroyed.
|Figure 5. Fossil occurrences in the K/T boundary interval, Wilderness Access Trailhead, BNP. The K/T boundary lies at the top of the Disrupted Zone (DZ). White arrow shows deformed sandy bed within the DZ. FZ _ zone containing bivalves, and rare osteichthian scales. CCT _ crustacean crawling traces. B _ belemnites.|
The DZ at this locality is located in Section 3, T2S, R15E about 0.5 km south of the Wilderness Access Trailhead parking area on Sage Rim Road. It can be accessed from a bison trail leading southward from the parking area. The DZ lies within the Fox Hills Formation and is about 5 m above the contact with the underlying Elk Butte Member of the Pierre Shale. As at Dillon Pass, and generally in Badlands National Park, the DZ interval lies within the brightly colored Interior Zone of Tertiary pedogenic alteration. The DZ is about 0.5 m thick at this site and is marked by convoluted sandy beds (Figure 5). At the base of the DZ is a thin, but distinct sheet of laminated sand, the base of which contains flute casts and drag marks indicating a southerly sediment transport direction. Directly beneath this sand sheet is a fine glauconitic siltstone, reddish at the top, which extends to the base of the outcrop. About 50 cm below the DZ is a thin horizon of lenticular, unfossiliferous ironstone concretions. The pre-DZ bedding sequence is thus similar to that occuring beneath the DZ at Dillion Pass. At the top of the DZ lies a second sand sheet, above which is a series of drab colored sand/shale couplets extending to the top of the outcrop. These beds become sandier upward. The base of the Chamberlain Pass Formation lies about 8 m above the DZ at the Wilderness Access Trailhead site.
Invertebrate and vertebrate fossils are preserved at Wilderness Access Trailhead. They are somewhat more abundant than at Dillon Pass, but preservation of body fossils is similarly poor, with severely leached impressions of isolated, and often fragmentary, skeletal elements most commonly encountered. Plant matter is only rarely seen at WATH. Destruction of organic remains due to Tertiary paleosol development thus seems to have been even more extensive here than at Dillon Pass. As at Dillon Pass, body fossils appear to be concentrated 20 _ 40 cm below the DZ (Figure 5), although belemnites occur lower still. No body fossils have yet been found within the DZ or above it at the WATH site. In contrast, trace fossils are abundant and found throughout most of the section, including the DZ.
Sage Creek Basin
Erosive down-cutting of Sage Creek and its headwaters in the interior of the
Sage Creek Wilderness area of Badlands National Park has incised deeply into
the upper Cretaceous units. Reconnaissance of this area reveals many exposures
of the Fox Hills Formation and DZ (Stoffer et al., 2001). Although this area
requires much further study, we include here preliminary observations from a
cutbank of the Middle Fork of Sage Creek located in section 29, T2S, R15E. Here
the Fox Hills Formation is about 8 m thick, and rests unconformably on the Elk
Butte Member of the Pierre Shale. The DZ is present in these Sage Creek Basin
localities and generally resembles the DZ at Wilderness Access Trailhead. The
fossil content of the DZ interval also appears similar to WATH, although no
detailed sampling has yet been attempted.
PALEONTOLOGY OF THE DISRUPTED ZONE
Fossils described here have been deposited in the collections of the Department of Invertebrates, American Museum of Natural History (AMNH), or the Department of Geology, Brooklyn College (BC) and are identified by AMNH, and BC catalog numbers. National Park Service catalog numbers (BADL) are given for specimens collected by permit within Badlands National Park, and temporarily housed at Brooklyn College prior to permanent deposition at AMNH. Although the K/T interval contains plant fragments, pollen and dinoflagellates, we discuss macrofauna and trace fossils only in this paper. Microfossils and plants will be dealt with in a later work.
The most abundant animals preserved in the Badlands K/T boundary interval are molluscs. Two classes are present: bivalves and cephalopods. Bivalves are numerous in the K/T sections; cephalopods are rare.
Molluscs occur at all of the K/T sections discussed
here, and at each one bivalves are numerically the most
abundant fossils present. However, it is not a diverse bivalve
fauna that one finds in these rocks. Only nuculids occur in
significant numbers, and only in the form of one
species: Nucula cancellata Meek & Hayden 1856. The only other bivalve
we have found to date is a poorly preserved specimen of
what probably is the inoceramid, Spyridoceramus
tegulatus (Meek & Hayden 1856).
Nucula cancellata Meek & Hayden 1856: Speden (1970) describes three congeneric species of Nucula from the Fox Hills Formation in its type area north and east of the Badlands: N. cancellata, N. planomarginata Meek & Hayden 1856; and N. percrassa Conrad 1858. All three species have the equivalve, inequilateral shell form and taxodont hinge structure typical of nuculids, but N. cancellata differs from the other two in having prominent radial costae on the outer surface of the shell, and strong crenulations on the ventral margin of the inner shell surface. In the Badlands area specimens illustrated in Figure 6, the taxodont hinge (Figure 6A, 6B), marginal crenulations (Figure 6A, 6B) and radial costae (Figure 6C) are apparent and identify these specimens as N. cancellata. Although most specimens recovered from these sections are more poorly preserved (Figure 6D), and show few or no diagnostic features, it seems apparent that virtually all bivalve material in these sections derives from this species. With one possible exception, discussed immediately below, no other bivalves have been unequivocally identified.
Speden (1970) indicates that in the Fox Hills type area, N. cancellata occurs in the Trail City and Timber Lake members, but not in the overlying Iron Lightning Member. These members are assigned an early late Maastrichtian age (Landman & Waage 1993). Thus, the occurrence of N. cancellata in the earliest Paleocene beds at Creighton (Figure 3) extends the stratigraphic range of this species significantly.
Figure 6. Nucula cancellata. A: AMNH-FI-46862; collected at Creighton, 20 cm below base of DZ. a _ anterior adductor muscle scar; h _ hinge teeth; c _ crenulations on inner surface of ventral margin. Scale bar = 1 cm. B: AMNH-FI-46862; close-up of posterior margin of the shell showing hinge teeth and crenulations on the ventral margin. p _ posterior adductor muscle scar. Scale bar = 0.5 cm. C: BC-CR008-00; collected within the DZ at Creighton showing radial costae on shell surface. Scale bar = 0.3 cm. D: BADL-20479, collected about 30 cm below DZ at Dillon Pass. Scale bar = 0.5 cm.
Figure 7: Spyridoceramus (=Tenuipteria) tegulatus. A: BADL-20468; poorly preserved, partial specimen of a possible S. tegulatus collected 10 cm below base of DZ at WATH. B: small "eared" form of juvenile S. tegulatus. From Plate 9, Figure 14, of Speden (1970). Scale bars in A and B = 0.5 cm.
Spyridoceramus (= Tenuipteria) tegulatus (Meek & Hayden, 1856): Examination of material collected at WATH, reveals a poorly preserved, tiny fragmentary shell (Fig 7A) that closely resembles the elongate, or "eared", juvenile form of the late Maastrichtian inoceramid, S. tegulatus (Fig. 7B). The WATH specimen is elongate, slightly curved, and shows the concentric ornamentation characteristic of the later forms of the species. The specimen also contains packets of fibrous material oriented perpendicular to the shell surface that we interpret as the leached remains of the carbonate fibrils that compose inoceramid shells. If we are correct in this assertion, then the implication is that Spyridoceramus ranged considerably higher in the Maastrichtian than other inoceramids (see MacLeod & Ward 1990).
Cephalopods are rare but important components of the Badlands K/T fauna. Two groups of cephalopods are present: scaphitid ammonites and belemnites. Also important is what cephalopods are not present _ baculitid ammonites. The absence of baculites from the K/T interval in the Badlands is consistent with the view of Cobban & Kennedy (1992) that these straight-shelled heteromorphs drop out of the record well below the end of the Cretaceous.
Scaphitid ammonites are present in small numbers in the Badlands K/T interval. They occur asrare, isolated fragments usually of the shell flank (Figure 8C, E); and as more complete juvenile shells (Figure 8D). Complete adult specimens (Figure 8A) are exceedingly rare. As a consequence, identification is difficult. Only two species have been identified so far. Further work may increase this number, but it remains to be seen whether the full complement of Maastrichtian scaphite species described by Landman & Waage (1993) from the Fox Hills type area also occurs in the Badlands.
Discoscaphites gulosus (Morton, 1834): The best preserved scaphite yet collected from these beds is the badly crushed, but nearly complete shell shown in Figure 8. The specimen has the adult hook and sutural morphology characteristic of scaphitids (Figure 8B). It is widely umbilicate, and strongly ribbed, with a row of prominent clavate tubercles along the ventrolateral margin of the shell, and a row of large tubercles on the umbilical shoulder. The shell flank also shows at least one row of tubercles. Based on the species definitions given in Landman & Waage (1993), these features, together with the relatively small size of the adult form, suggest that this specimen is a microconch of Discoscaphites gulosus. The specimen shown in Figure 8C, although only a small shell fragment, shows two rows of prominent tubercles with each tubercle pair centered on a rib. Ornament of this type is also typically seen in D. gulosus.
Jeletzkytes nebrascensis (Owen, 1852): The shell fragment shown in Figure 8E represents a part of the flank of a scaphite shell. Judging from the curvature of the specimen, the fragment extends from about the umbilical shoulder to the ventral shoulder (top to bottom in Figure 8E). The fragment has closely spaced, slightly sinuous and prorsiradiate ribs (about 8 ribs/cm counting along the shell spiral). This suggests that the fragment is from the body chamber of an adult shell. If this is true, then the size of the fragment would indicate that the entire shell probably had a diameter of about 8 to 10 cm. The ribs are of moderate height with most ribs extending from the umbilical to ventral shoulder. Intercalated among the long primary ribs, are several shorter secondary ribs. In addition, small circular tubercles (indicated by white arrows in Figure 8E) are located at intervals along the length of many of the ribs. Part of one large clavate tubercle, can be seen at the ventral shoulder just to the right of the scale bar in Figure 8E. Although it is difficult to determine for certain in so small a fragment, there is some indication that the tubercles are arranged in rows. These features of ribbing and tuberculation, when referenced to species diagnoses given by Landman & Waage (1993) for Fox Hills scaphites, indicate that the specimen is from the shell of Jeletzkytes nebrascensis. The estimated diameter of 8 to 10 cm, together with the tuberculation, indicate it is from a small macroconch of this species. Presumably, the original shell resembled YPM23145, a specimen held at Yale Peabody Museum, and illustrated by Landman & Waage (1993, Fig. 123A-D).
J. nebrascensis is among the most common scaphites of the Timber Lake Member in the Fox Hills type area and in adjacent parts of North Dakota (Landman & Waage 1993). That it occurs in the Badlands K/T boundary interval, even as a rarity, indicates that its stratigraphic range extends considerably higher than previously documented. J. nebrascensis ammonite range zone. Thus, the upper boundary for this late Maastrichtian biostratigraphic zone, should be extended upwards tot he K/T boundary.
|Figure 8. Scaphitid ammonites from the Badlands K/T boundary interval. A: adult microconch of Discoscaphites gulosus (AMNH-FI-46861) collected within the DZ at Creighton. u _ umbilicus; t _ tubercle rows (only last tubercle in each row is indicated). Scale bar = 2 cm. (from Figure 2F of Terry et al., 2001). B: enlargement of phragmocone of AMNH-FI-46861 showing partly exposed suture lines of typically scaphitid form. Scale bar = 0.3 cm. C: shell fragment (BC-CR016-00), showing two rows of large tubercles, with pairs of adjacent tubercles positioned on the same rib. Specimen collected 20 cm below DZ at Creighton. Scale bar = 0.5 cm. D: heavily ribbed juvenile scaphite (AMNH-FI-46860) collected 10 cm below the top of the DZ at Creighton. Scale bar = 1 cm. (from Fig 2G of Terry et al. 2001). E: fragment of shell flank of Jeletzkytes nebrascensis; (BADL-20479), collected 30 cm below DZ at Dillon Pass. Arrows point to small isolated tubercles characteristic of the species. Scale bar = 0.7 cm.|
We have collected belemnite specimens in the beds below the DZ at the Wilderness Trailhead locality. None occur higher than about 1 m below the base of the DZ. All appear to be Belmnitella americana Morton 1834, the common Western Interior species. The specimens are of the solid, tapering part of the rostrum, and show the radial carbonate prisms characteristic of this structure.
|Figure 9. Osteichthian fossils in K/T boundary interval. A: osteichthian scale (BADL-20529) collected 40 cm below base of DZ at WATH. Scale bar = 0.5 cm. B: enlargement of A showing crenulated surface ornament of scale. Scale bar = 0.25 cm.|
Vertebrate remains are uncommon constitutents of the four Badlands K/T localities discussed here. Osteichthean scales (Figure 9) have been found above, below, and within the DZ at Creighton and they have been found below the DZ at Wilderness Access Trailhead. Chondrichthian remains are also present. A few teeth of lamniform sharks have been recovered from within and above the DZ at Creighton (Figure10A). None of the teeth retain roots, so that it is not yet possible be more precise in their identification. In addition, we have recovered the partial skeleton from Sage Creek Basin of what we interpret as some type of small chondrichthian. This specimen was found as float in the stream bed of the Middle Fork of Sage Creek at the site noted in Figure 2, and discussed above. It undoubtedly weathered out of the adjacent cutbank which exposes late Cretaceous and lower Tertiary rocks. The specimen is preserved in a lenticular concretion of Fox Hills color and character, but the original relation of the specimen to the DZ can not be ascertained. Since the Fox Hills is about 8 m thick at this site, the specimen originally could not have been more than a few meters above or below the DZ, if not originally preserved within it.
As seen in Figure 10B, only a portion of the vertebral column and pectoral
girdle, and one of the pectoral fins is preserved. Head and tail are missing.
Skeletal elements are dark or amber colored and appear translucent in strong
light. We consider the specimen to be a chondrichthian on the basis of several
characters:1) the vertebrae are simple with a central passage for the notochord
that is large relative to the size of the vertebrae; and 2) there is no indication
of nueral arches, or other osteichthian vertebral structures. We are uncertain
as to what kind of a chondrichthian this is, but the elongate body would probaly
rule out batoids. Our current view is that it is a small, neoselachian shark,
but further anatomic analysis is required to fully idnetify this animal.
|Figure 10. K/T boundary interval chondrichthians. A: lamniform shark tooth from within the DZ at Creighton. Scale bar = 0.5 cm. B. Partial chondrichthian skeleton (BADL-20530) from Sage Creek Basin. v _ vertebral column; p _ pectoral girdle; f _ pectoral fin. Scale bar = 3 cm. C: enlargement of vertebral column showing three vertebrae. n _ notochord. Scale bar = 0.4 cm. D: proximal region of pectoral fin. Scale bar = 0.5 cm.|
Bioturbation is a common feature of the sandier beds in all of the Badlands K/T sections discussed here. Trace fossils are also numerous at Wilderness Access Trailhead and Creighton. For the most part, trace fossils found at these two localities belong to the same ichnogenera common in Cretaceous rocks elsewhere in the Western Interior. Ophiomorpha, Climacodichnus and Diplocraterion-like burrows are particularly prominent in sheet sands at these sites. This is especially true at WATH, where the distorted sandy beds within the DZ and the sand sheet marking the top of the DZ show profuse Climacodichnus bioturbation.
Figure 11. K/T interval trace fossils and probable modern analogs. Scale bar = 3 cm in A, B, C, E, F. A, B: feather-like trackways (BADL-20500), collected about 3 m above the DZ at WATH. A: two trackways (1 and 2) on bedding surface of fine grained sandstone. White arrow _ degassing bubble crater. B: enlargement of trackway 2 in A. C: trackway of modern nearshore isopod Oniscus. (from Fig 127.5 of Hantzschel, 1962). D, E, F: burrowing crab (Emerita) and trackways from swath zone of ocean-facing beach at Island Beach State Park, NJ. C: Emerita, head at right; tail at left. Total body length _ 3 cm. D: animal begins to burrow: E: animal descends below sand surface.
In addition to these trace fossils, the WATH section preserves many examples of the feather-like ichnofossil illustrated in Figure 11. These structures occur below and within the DZ, but become more numerous, and better developed, above the DZ, as the section becomes sandier (Figure 5). Superficially, they resemble the crawling traces made by isopod crustaceans in having an elevated central axis, and scour marks arranged in en echelon fashion along the central axis and aligned at acute angles to it (Figure 11C). However, the Badlands ichnofossils differ from isopod trackways in being relatively short, straight structures, rather than the elongate, curved, and often sinuous, trails made by these scavenging epifaunal crustaceans. In addition, isopod trails do not commonly show the elevated periphery present in the better preserved Badlands specimens (Figure 11B).
Although isopods cannot be ruled out as the makers of the Badlands "feather" trails, the identity of the Badlands track maker may lie in another group of benthic crustaceans _ anomuran crabs. Figure 12D shows Emerita, a burrowing anomuran crab of the New Jersey coastline, known to local saltwater anglers as "mole crab". Emerita inhabits the shifting sand of ocean facing beaches, where in the summer it is commonly seen near high tide lines frantically digging back into the sand when exhumed by waves. Figures11E and 11F, show a trackway made by a burrowing Emerita. As the animal begins to descend into the sand, it plows up a ridge of sand around its body and the rowing action of its appendages, which push the animal into the sand, produce en echelon sand ridges aligned along the axis of the trackway. Because Emerita descends into the substrate within one or two body lengths, its trackway on the sand surface is never more than a few centimeters at most. The result is a structure very much like that of the feather-like Badlands ichnofossils.
In addition, feather trails are most abundant in the upper, sandy part of the WATH exposure, and are invariably associated in these upper sand beds with small circular pits (Fig 11A). These pits taper downwards into the underlying sediment, and are often surrounded by a small raised rim of mounded sand grains. Although they could be incipient cone-in-cone structures, the pits do not show any sign of the chevron laminae characteristic of these sediment dewatering structures. However, they do resemble very closely, particularly with respect to the mounded sand rims, degassation pits that develop in the swath zone of modern beaches, as air trapped interstitially by wave surge migrates upwards and then explosively escapes at the beach surface. The association of the Badlands trails with such structures suggests that the Badlands trackmaker inhabited shifting beach or tidal channel sands as does modern Emerita. This strengthens the interpretation of these trails the burrows of this burrowing decapod crustacean.
The four K/T boundary localities discussed here preserve invertebrate, vertebrate, and plant remains. Study of this material indicates that Badlands fossils belong to species well known in upper Cretaceous rocks of the Western Interior. But the broad diversity of taxa characterizing the late Cretaceous Western Interior marine fauna does not occur int he Badlands K/T interval. Certainly, the species-rich concretionary horizons of the Fox Hills type area described by Waage (1968), Speden (1970) and Landman & Waage (1993) are absent in the Badlands. Instead, one finds at most a few species at a given site, although sometimes in significant numbers. N. cancellata, and the feather-trail trackmaker are examples.
To some extent this diminished diversity might reflect destruction of Badlands fossils due to later pedogenic alteration. But this is an unsatisfying explanation because the Creighton section was not affected by paleosol development, and it has a restricted taxonomic diversity little different from the Badlands sites. Moreover, species comprising the fossil-rich sites in the Fox Hills type area, but not occurring at the Badlands sites, have shells that are no more susceptible to geochemical degradation than those species that do occur in the Badlands area. If a wide array of type area species inhabited the Badlands area during the K/T transition, most of these species should still be there, perhaps altered and leached, and reduced in population size, but still present. Thus, we suggest that there is a real ecologic signal coming through the geochemical filter of Interior Zone pedogenesis that reflects original distribution and abundance. This signal indicates that the K/T boundary interval was a time of highly restricted diversity in the Western Interior fauna _ at least as we know it from the Badlands. Furthermore, this low diversity is inherently related to conditions within the Seaway. It has nothing to do with the Chicxulub impact because the study localities are faunally impoverished below the impact horizon at the top of the DZ.
Among species preserved in the study area, there are broad differences in numerical abundance between the collecting localities, which are apparent even though we have not yet quantified them. The feather-trail trackmaker is fairly common at Wilderness Access Trailhead, but not found elsewhere, for example. In contrast, nuculids are more abundant at WATH than they are at Dillon Pass, while for scaphites, the situation is reversed. The Grassy Tables Overlook site has no body fossils at all.
To some extent these abundance patterns are undoubtedly the product of later Interior Zone alteration, which varies in intensity across the study area. Thus one would expect carbonate shells to be less abundant in the heavily altered Badlands sites than at Creighton - which they are. But such an explanation does not hold for the feather-trail track maker. Nor can it explain faunal differences among Badlands National Park sites, where the geochemical signature is of roughly equivalent strength. Again, an original ecologic signal is coming through, a signal pointing to local differences in environment and in the distribution patterns of the animals inhabiting the region during K/T transition.
Some understanding of these matters can be gained by reflecting on the paleogeography of the Badlands area during the K/T interval. The core of the issue is the Sage Creek Arch, an anticlinal structure extending roughly NW/SE through the north unit of Badlands National Park. The Badlands K/T sites sit on the southern flank of this structure, near its crest, while Creighton lies to the north. It is evident from the occurrence of intra-formational unconformities and erosive hiatuses within the Fox Hills Formation and the Pierre Shale, as well as from the thinning of the Cretaceous beds over the arch, that the structure was active during the K/T transition (Stoffer, 1998; Stoffer et al., 1998; Stoffer et al., 2001; Terry et al., in review). At times, the crest of the arch was raised above the surface of the Seaway. The discovery of root traces at the top of the Dillon Pass DZ (Terry et al., in review) indicates that the crest was at least partly exposed at the end of the Cretaceous.
Thus, we envision the Badlands area at the K/T boundary to have been a low-lying, sandy peninsula or a series of low sandy islets surrounded by intervening sandy lagoons and tidal channels. The whole structure extended southeastward into the deeper waters of the Western Interior Seaway just offshore. Because the end-K Seaway was smaller, and presumably more restricted oceanographically, than earlier in the Cretaceous, it may have been more inhospitable to stenohaline and stenothermal marine animals, and perhaps less stable environmentally. If so, this would have had an important effect in reducing species diversity at the end of the Cretaceous. The outcrops from which we now collect our K/T fossils were sites of sediment accumulation in the shallow, ephemeral waterways of the crest. Such environments today are often associated with an impoverished macrofauna, in which the few successful forms exist in great numbers. Thus, the prime hallmark of the Badlands during the K/T interval, the impoverishment of its fauna, may have derived from a combination of regional oceanographic factors coupled with local high stress environments. Superimposed on all this in the DZ and the beds immediately above, is the environmental fall-out of the Chicxulub impact.
In the end, how much of this scenario proves to be correct depends very heavily on further research. But the idea that the end of the Cretaceous was an inhospitable time for the biota of the Western Interior seems evident.
THE BADLANDS K/T INTERVAL AND THE END-K MASS EXTINCTION
When Alvarez et al. (1980) suggested that bolide impact was the cause of the terminal Cretaceous mass extinction, they set off a vociferous debate on this subject that continues still. In this debate, attention has centered on dinosaurs and ammonites, the two groups of macroscopic animals whose final extinctions are linked most prominently to the end of the Cretaceous. Much interest has focused on exactly when these lineages terminate relative to the K/T impact horizon. For ammonites this is not exactly clear (Ward 1990), although in most cases, as for example the K/T boundary sections in northern Spain (Ward et al., 1986), ammonites appear to drop out of the record well below the K/T boundary.
The Badlands DZ represents soft sediment deformation due to shock waves emanating from the Chicxulub impact (Stoffer et al. 2001; Terry et al., in review). Thus, in the Badlands the K/T boundary is defined as the horizon separating the top of the DZ from the overlying undisturbed beds (the first one of which at Dillon Pass contains impact ejecta). At Creighton, scaphitid ammonites are preserved within the upper 10 cm of the DZ _ within 10 cm of the K/T boundary. Stratigraphic distortion of DZ beds not withstanding, this makes the Badlands scaphites among the youngest ammonites ever reported. But, does this mean that these scaphites survived up to the K/T boundary and that they were indeed victims of the Chicxulub impact? It is too early to say. The distortion of DZ beds; the paucity of scaphites within the DZ; and the ever-present problem of the Signor-Lipps effect (Signor and Lipps, 1982) make a firm conclusion on the exact stratigraphic position of the "last" Badlands scaphites premature. Moreover, the data, as they now exist, also support the view that the Badlands scaphites succumbed slightly before the end of the Cretaceous. If this were the case, then factors other than impact were decisive. In this alternative scenario, paleoenvironmental change deriving from the latest Maastrichtian sea level low stand (Haq et al., 1987), which some advocate to have occurred about two hundred thousand years prior to the K/T transition (Stinnesbeck & Keller, 1996), offers a plausible alternative explanation for the extinction of the Badlands scaphites. Further collection work with a focus on microfossil, as well as scaphite, biostratigraphy should prove useful in resolving this issue.
We thank A. Chamberlain, L. Factor, P. Jannett, P. Messina, J. Mundt, and D. Patrick for help in the field. R. Benton, Paleontologist at Badlands National Park, provided research permits and advice on field logistics. N. Landman assisted in identification of the scaphitids; and T. White and C. MacClintock assisted in our study of Fox Hills specimens held at the Yale Peabody Museum. This research was supported by grants to JAC from the PSC-CUNY Research Award Program of the City University of New York, and NSF (STI-9602595); a grant to JAC and PWS from the EDMAP program of the USGS; and a grant to DOT from the College of Science and Technology at Temple University.
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