the cretaceous/paleogene boundary events in the gulf coast

The Cretaceous/Paleogene Boundary Events in the Gulf Coast: Comparisons between Alabama and Texas

Malcolm B. Hart1, Peter J. Harries2, and Andrés L. Cárdenas3

1School of Geography, Earth and Environmental Sciences, Plymouth University, Drake Circus, Plymouth PL4 8AA, United Kingdom

2Department of Geology, University of South Florida, 4202 E. Fowler Ave., Tampa, Florida 33620–8100

3Center for Tropical Paleontology and Archaeology,

Smithsonian Tropical Research Institute, Balbao, Ancon, 0843–03092, Panamá

ABSTRACT

Following identification of the Chicxulub crater on the Yucatan Peninsula, the Cre-taceous/Paleogene (K/Pg) boundary sections in the Gulf Coastal Plain were recognised as being of international significance. In the period 2009–2012, M. B. Hart and co-workers have investigated a number of successions in the Brazos River area, Falls County, Texas, and reported on their sedimentology, micropaleontology and geochronology. Compara-ble sections in Alabama have received relatively little attention in recent years and this is now being addressed.

Sections on Highway 263 near Braggs, Mussel Creek and Moscow Landing appear to have been deposited in shallower-water environments as compared to those in Texas. The Braggs section, which is often regarded as the ‘type’ section by workers in the area, is—in fact—rather atypical. Above a sharp erosive surface at the top of the Prairie Bluff Chalk Formation (in a mudstone facies), the majority of the overlying strata are con-cordant with few signs of hiatuses at lithological boundaries. Many of the carbonate-rich beds within the mudstones of the Clayton Formation (Pine Barren Member) of the Lower Paleocene appear to be diagenetic in origin. In the Mussel Creek succession, which is relatively close to the Braggs section, there are numerous hiatuses detected, many of which show complex bioturbation. Above the almost flat, but bioturbated, top of the Prairie Bluff Chalk Formation is a thin, complex succession of sandstones, clay-stones, eroded blocks of the Prairie Bluff Chalk Formation and prominent fragments of charcoal. This complex series of ‘events’ is followed by a carbonate-rich bed with bro-ken shell fragments and phosphatic clasts, including ammonite fragments, above which is a series of Paleocene mudstones and carbonate-rich beds comparable to those record-ed in the Braggs section. The Moscow Landing succession, exposed on the west bank of the Tombigbee River, is located approximately 135 km west-northwest of the Braggs area. In this succession the Prairie Bluff Chalk Formation (in a more carbonate-rich facies) is overlain by a brown, coarse, spherule-bearing sandstone that is associated with a small number of widely-spaced, internally complex, channels before—in turn—being overlain by a succession of mudstones and carbonate-rich beds. We identify two region-ally important events in these three locations. The top-Prairie Bluff Chalk Formation surface is smooth, but undulating, and appears to be the result of regional erosion at the time of the Chicxulub impact. Between this surface and the local base of Zone P1a, the Clayton (Basal) Sands contain an admixture of sediments, re-worked clasts of the under-lying Prairie Bluff Chalk Formation, re-worked macrofossils, re-worked microfossils and (in the west of Alabama) re-deposited spherules. We have been able to piece togeth-er the sequence of events in all these sections and linked our preliminary interpretations to both the now-buried Miller’s Ferry sections and the Brazos River area.

Hart, M. B., P. J. Harries, and A. L. Cárdenas, 2013, The Cretaceous/Paleogene boundary events in the Gulf Coast: Comparisons between Alabama and Texas: Gulf Coast Association of Geological Societies Transactions, v. 63, p. 235–255.

Hart et al.

INTRODUCTION The Cretaceous-Paleogene (K/Pg) boundary still commands global attention as geoscientists, of all disci-

plines, search for an all-embracing model to explain this significant extinction event (see Alegret et al., 2011, for a recent review). While distal areas, well away from the Chicxulub ‘impact’ site, provide an accurate biostratig-raphy and cyclostratigraphy for the timing of the K/Pg events, the proximal sites (Mexico, Cuba, Texas, and Ala-bama) provide detailed information on the impact history. Recently, Hart et al. (2012) have attempted to resolve some of the events recorded in the Brazos River area of Falls County, Texas. In that area the K/Pg boundary successions were described as being located in a mid-shelf environment with relatively little fluctuation in sea levels. Yancey and Liu (2013) have presented a water-depth analysis of the uppermost Maastrichtian and lower-most Paleocene sediments and given a range of 50–150 m, with some evidence of fluctuating sea levels around the K/Pg boundary itself. With so much upheaval in the microfossil planktic and benthic communities (foraminifera, ostracodes, calcareous nannofossils, etc.) finding a reliable guide to water depth changes and eco-system disturbance is both complex and difficult.

The K/Pg boundary successions in Alabama are regarded as having been deposited in a shallower-water environment than those in Texas and with a thinner sedimentary cover overlying the Paleozoic basement (Donovan et al., 1988; Zachos et al., 1989; Mancini and Tew, 1993; Olsson and Liu, 1993; Savrda, 1993; Smit et al., 1996). Many of the descriptions of these successions (Baum and Vail, 1988; Donovan et al., 1988; Savrda, 1993) have applied sequence stratigraphic principles to the interpretation of these sediments. While such models are successful at the identification of “genetically related strata bounded at its top and base by unconformities or their correlative conformities” (Mitchum, 1977; Van Wagoner et al., 1988; Donovan et al., 1988, p. 299), they may be less adept at the identification of an instantaneous event (and its aftermath). This paper assesses what has been learned from other areas (Texas, Denmark, Demerara Rise, etc.) and attempts to identify both similarities and differences.

LOCATIONS The three successions described in this review are located (Fig. 1) in mid-central Alabama and include: 1. Braggs—on both sides (northeast and southwest) of Alabama State Highway 263 west of Greenville and

7.4 km southeast of Braggs (32.006°N, 86.751°W); 2. Mussel Creek—just north of Highway 263 and ~3.2 km from the Braggs succession (31.9717°N,

86.7043°W); and 3. Moscow Landing (Sumter County)—on the west bank of the Tombigbee River south of the now-

dismantled Rooster Bridge (32.4263°N, 88.0386°W). The Miller’s Ferry location, described by Olsson et al. (1996) is now a working hydro-electric station and all

the successions described during construction (1966–1967) of the power plant are now buried. That site investi-gation does, however, provide a great deal of valuable information.

THE CRETACEOUS-PALEOGENE BOUNDARY While many of the earlier publications on the successions in Alabama have agonized over the placing of the

K/Pg boundary and the question of re-worked or in situ fossils, the official definition by Molina et al. (2006) indi-cates that the base of the Paleocene is marked by the first (sedimentological) evidence of the Chicxulub impact event and that all the overlying sediments are, therefore, of Paleocene age. This definition has significant impli-cations for the interpretation of the Alabama successions and the events that occurred as a result of the end-Cretaceous impact on the Yucatan Peninsula.

Braggs Section (32.006°N, 86.751°W) This series of exposures, which can be combined (Fig. 2) into a composite section, are located on Alabama

State Highway 263 in Lowndes County, 7.4 km southeast of Braggs and northwest of Greenville. The Creta-ceous-Paleogene (K/Pg) boundary section (Fig. 3) was supplemented, in October 2012, by a recent slump which increased exposure ~100 m to the southeast (Fig. 4) and which could also be linked to a further exposure on the opposite side of the road. At the base of the exposed succession, which usually requires excavation (Fig. 3), the

Prairie Bluff Chalk Formation (PBC) is a dark gray to almost black (when wet) siltstone to sandy, micaceous, calcareous-rich mudstone. Some glauconite is present and the lack of visible sedimentary structures is suggestive of pervasive bioturbation. Disarticulated molluscan shells are present, with a slight concentration near the base of the exposed section in Figure 3 (base of tape measure). Cepek et al. (1968) indicated that the lack of the planktic foraminiferal species Abathomphalus mayaroensis indicated that the PBC was Middle Maastrichtian (Gansserina gansseri Zone) in age and that there was a significant hiatus at the top of the PBC. In contrast, Zachos et al. (1989) suggested that the absence of this relatively deep-water taxon may reflect the relatively shallow deposi-tional environment which Olsson and Liu (1993) and Olsson et al. (1996) confirmed. The presence of a planktic foraminiferal assemblage dominated by heterohelicids (including Planoglobulina carseyae, Pseudotextularia deformis, Racemiguebelina fructicosa and Ventilabrella multicamerata) and species of Rugoglobigerina is highly reminiscent of the uppermost Maastrichtian in the Stevns Klint succession of Denmark (Hart et al., 2004, 2005). Habib et al. (1992), using the paleontological record of dinoflagellates cysts, also indicate that the uppermost Maastrichtian (Micula prinsii Zone) is present in the Braggs section.

The PBC is terminated by a sharp, erosional, boundary that appears planar, although the limited extent of the exposure precludes a full assessment of this surface’s lateral persistence. At Mussel Creek (see below), the top of the PBC is again erosional with a smooth, undulating, topography as may also be the case in the Braggs section. The basal, carbonate-rich, bed of the Pine Barren Member (PBM) of the Clayton Formation contains abundant shell fragments, quartz grains, re-worked Cretaceous fossils and rounded, dark-brown phosphate clasts. This bed, which may be described as a calcarenite, contains rare grains of glauconite that are sometimes weathered to Fe-oxide. Above this distinctive bed there appears to be a conformable succession of dark mudstones (often contain-ing glauconite), siltstones and prominent carbonate-rich beds (Fig. 4), many of which appear to be diagenetic in

Figure 1. Map of Alabama, which shows the location of the sections discussed in the text.

origin. Weathering of the glauconite to orange Fe-oxides in the upper levels of this exposure have, again, discol-ored the succession.

The PBM sediments overlying the basal calcarenite contain Paleocene planktic and benthic foraminifers in association with Paleocene molluscs. Cepek et al. (1968) did not record the presence of the lowermost Paleo-

Hart et al.

Figure 2. Composite sedimentary logs of the sections discussed in the text. Scale in meters.

Figure 3. The Cretaceous/Paleogene boundary in the Braggs succession on Highway 263 west of Greenville. The upper surface of the Prairie Bluff Chalk Formation is overlain by a prominent bed of calcarenite which forms the local base of the Pine Barren Member (Clayton Formation).

cene, although both Worsley (1974) and Thierstein (1981) recorded calcareous nannofossil zones NP1 and NP2. The K/Pg boundary lies at the eroded surface of the PBC (following Molina et al., 2006), although many previous workers have placed it at a higher level (Copeland and Mancini, 1986; Jones et al., 1987; Donovan et al., 1988; Zachos et al., 1989).

The identification of a major sequence boundary at the top of the PBC by Donovan et al. (1988) and subse-quent workers has created difficulties as this ‘event’ has then been correlated with global records (Haq et al., 1987, 1988). The top-PBC burrowed, erosional surface is clearly significant and a regional unconformity (Donovan et al., 1988, their figs. 2 and 3) but is here recognised as the K/Pg event. The stable isotope and geo-chemical data provided by Zachos et al. (1989) must also be re-interpreted to the revised K/Pg boundary. While some of the elemental changes appear un-related to any particular boundary, some of the recorded changes do appear to look more appropriate against the revised boundary. The stable isotope data are quite noisy, although the re-adjustment would place a negative δ13C excursion just above the boundary, although this should probably not be equated with a similar, but much larger, excursion recorded in the successions at El Kef (Molina et al., 2006) and Stevns Klint (Hart et al., 2005, their fig. 10).

The previous interpretation of a major sequence boundary coincident with the upper surface of the PBC re-lied on a major sea level change and the presence of small, incised channel-fill deposits at Mussel Creek

Hart et al.

Figure 4. The Pine Barren Member (Clayton Formation) in the Braggs section exposed in a temporary exposure in October 2012. The boundary section (Fig. 3) is below, and to the right of, this section. Many of the carbonate-rich beds within the Pine Barren Member appear to be diagenetic in origin.

(Donovan, 1986; Donovan and Vail, 1986), Miller’s Ferry (LaMoreaux and Toulmin, 1959) and Moscow Land-ing (Smith et al., 1984). Donovan et al (1988) and Mancini et al. (1989) noted that none of these sand bodies were present in the Braggs succession.

Mussel Creek (31.9717°N, 86.7043°W) This stream-cut exposure is on the west bank of a northeast trending segment of Mussel Creek in southern

Lowndes County (Fig. 1). The K/Pg boundary succession begins under the road bridge across the creek (Fig. 5) and extends ~100 m to the north (Fig. 6). The higher levels of the succession are located in the road cuts as one approaches the creek from Highway 263 (Fig. 7). This exposure of the Pine Barren Member (Clayton Formation) comprises ~10 m of alternating calcareous mudstones and irregular, nodular limestones. This succession is the equivalent of those seen in the Braggs section (Fig. 4) and the continuation of this succession seen on the opposite side of Highway 263. A similar series of cyclical mudstones and nodular limestones were recorded at Miller’s Ferry by Olsson et al. (1996, their figs. 4, 5B, 6A, 6C, and 8A)

The Mussel Creek section has been described by a number of workers (Donovan et al., 1988; Mancini et al., 1989; Habib et al., 1992; Savrda, 1993). Unfortunately the placing of the K/Pg boundary has, again, been an issue, as has the application of sequence stratigraphy to the succession. Savrda’s (1993) analysis encompassed the most detailed description of the succession, although the motivation for his research appears to have been to repudiate the Chicxulub impact and this may have led to a biased view of the sedimentology.

Despite the proximity to the Braggs succession, the events recorded here appear quite different. The PBC is again represented by dark, glauconitic mudstone and siltstone with pervasive bioturbation (including Zoophycos, Planolites, and Thalassinoides) and with an identical microfossil assemblage to that recorded in the Braggs suc-cession. Immediately below the bridge (Fig. 5), the PBC is overlain by a prominent, calcareous-rich bed that contains shell fragments, quartz grains and rounded phosphate clasts. This appears to be the equivalent of the bed seen immediately above the PBC in the Braggs succession (Fig. 3). Approximately 15 m north of the bridge (Fig. 5) the eroded top of the PBC is overlain by a yellow-colored, mottled mudstone that is, in turn, overlain by the calcarenite bed. The upper surface of the PBC is smooth (Figs. 5 and 8) but displays undulations of ~1 m ampli-tude, which are comparable to those recorded at Miller’s Ferry by Olsson et al. (1996, their fig. 11). This bound-ary, which was described as ‘Surface A’ by Savrda (1993, his figs. 1 and 3) is clearly erosional but the smooth-ness does not suggest that it was formed in a sub-aerial or channel-side environment. There is no evidence of paleosol formation or the style of scouring that would indicate erosion in a channel. The topography and general nature of the top-PBC surface is entirely reminiscent of the eroded surface of the Corsicana Mudstone Formation in the Brazos River area of Texas (Hart et al., 2012, their fig. 4).

In the center of the Mussel Creek exposure is an area described in great detail by Savrda (1993, his fig. 3) and which is shown in Figures 9 and 10. The yellow-colored mudstones, siltstones and sandstones that overlie the top-PBC surface are internally laminated, graded in places and markedly cross-bedded. They contain large clasts of PBC (Fig. 9, adjacent to the ruler), clasts of hardened mudstone and fragments of charcoal. Some of these plant fragments are quite large, with stems of ~10 cm diameter, dense black and potentially showing growth rings. Previous descriptions of the Clayton (Basal) Sands have mentioned the ‘carbonaceous’ character of the sediments but there is no direct mention of charcoal fragments.

Savrda (1993) identified two surfaces (A and B) that form distinctive and important boundaries within the succession. Surface A is the regional erosion surface at the top of the PBC and, following the definition of Moli-na et al. (2006), represents the K/Pg boundary. Surface B, which forms the base of the Pine Barren Member (PBM), is also an erosive boundary and can be traced all along the exposure in Mussel Creek. It also appears to be the boundary between the PBC and the PBM in the Braggs section. Within the intervening yellow-colored mudstones, siltstones and sandstones of the Clayton Basal Sands are the ‘exotic’ blocks described by Savrda (1993, his fig. 3, bed 6A). Figure 9 shows exactly the same area with the smooth, but undulating, top-PBC sur-face as the prominent feature. The calcarenite at the base of the PBM forms the overhanging ledge above the Clayton Basal Sands. The enclosed block of dark-colored PBC mudstone, clearly illustrated by Savrda (1993), is near the ruler: this is also seen in Figure 10. Savrda (1993, his fig. 3, bed 6A) indicates that all of the enclosed blocks were the same material but this is not the case. The small block is dark PBC mudstone, but the larger blocks (Fig. 10) are more hardened and sit within the Clayton Basal Sands, with yellow-colored sands containing abundant plant debris both above and below. The shell-rich calacrenite with phosphate clasts and re-worked Cre-taceous fossils seen in both the Mussel Creek and Braggs successions is less prominent in the Miller’s Ferry suc-cession, although Olsson et al. (1996, their fig. 7) show the boundary between the Clayton Basal Sands and the overlying Pine Barren Member as a transgressive surface with intense bioturbation.

At Miller’s Ferry, Olsson et al. (1996, their figs. 7 and 11) described a ‘Chaotic Bed’ at the base of the Clay-ton Basal Sands: it was also named the K/T Event Bed. The ‘Chaotic Bed’ contained blocks of re-worked PBC and rests on a thin clay horizon. The overlying, thinly-bedded micaceous siltstones and sandstones contain early Paleocene planktic foraminifera (Olsson et al., 1996, their fig. 4) and on-lap ‘highs’ in the top-PBC surface. Within the ‘Chaotic Bed’ are rare, scattered, calcite-replaced spherules (Olsson et al., 1996, p. 269) but there does not appear to have been a spherule layer as described in the Brazos River area of Texas by Hart et al. (2012, their fig. 4). Spherules are, reportedly, more abundant at Shell Creek (Wawak, 2007, p. 70–74) which is located only a few kilometres from the Miller’s Ferry construction site.

The Braggs and Mussel Creek successions can be used to demonstrate a smoothed, but undulating, top-PBC, which is again recorded at Miller’s Ferry and other locations such as Shell Creek (Wawak, 2007). The Clayton Sands at Mussel Creek and at Miller’s Ferry are resting on this surface, with both containing ‘chaotic’ blocks of re-worked PBC. Olsson et al. (1996, p. 269) also indicated the presence of rare, scattered, calcite-replaced spher-ules in this bed which indicate that it is post-impact and that the top-PBC is the eroded surface that marks the K/Pg boundary. To the north of Braggs, Mussel Creek and Miller’s Ferry (and Moscow Landing) the uppermost Maastrichtian is represented by a siliciclastic unit, the Providence Sand Formation (Donovan et al., 1988, their figs. 2 and 3; Olsson et al., 1996, their fig. 12) and that the Clayton Basal Sands may have been derived from the north. Olsson et al. (1996) indicate that the sand-rich Clayton Basal Sand may have been a backwash from the K/

Hart et al.

Figure 5. The Mussel Creek section begins under the road bridge (left) and extends over 100 m along the creek (behind camera). Immediately under the bridge, the base of the Pine Barren Member (Clayton Formation) rests directly on the uppermost Prairie Bluff Chalk Formation. Near the pick, the smooth but undulating top PBCF surface is overlain by pale yell, sandy mudstone that underlies the calcarenite at the base of the PBM.

Pg tsunami (Olsson et al., 1996, p. 272), although its appearance in Mussel Creek suggests non-instantaneous deposition.

In the Braggs and Mussel Creek sections, we recorded no evidence of faulting, although the exposures are relatively limited. Detailed mapping of the top-PBC surface in the Miller’s Ferry construction site (Olsson et al., 1996, their fig. 2) showed the presence of small faults, all of which were oriented north-south. The ‘crest feature’ has an orientation at 90º to this trend, being almost east-west. In a major site investigation and construction site, even the smallest of faults are to be recorded, and it is unsurprising that comparable features were not seen at Braggs and in Mussel Creek.

At Shell Creek (LaMoreaux and Toulmin, 1959; Jones, 1967; Mancini et al., 1989; Smit et al., 1996; Bralower et al., 1998; King and Petruny, 2003, 2004, 2005; Wawak, 2007) the intensely bioturbated PBC is over-lain by a ‘Basal Clayton Sandstone’. The lowermost, brown layer is a coarse sand with rare spherules, re-worked Cretaceous fossils, shark teeth and a mixture of re-worked microfossils. Spherules become more abundant a few centimeters above the lower, Fe-stained sands. King and Petruny (2005) indicated that the microtektites of Shell Creek represent the most easterly occurrence of Chicxulub impact ejecta in Alabama, although they were also recorded at Miller’s Ferry by Olsson et al. (1996). The top-PBC surface is described as showing channel-like depressions that are in-filled by sands, shell fragments, PBC clasts, and re-worked microfossils (Smit et al., 1996; Wawak, 2007, p. 71–73).

Moscow Landing (32.4263°N, 88.0386°W)

This exposure, which is on the west bank of the Tombigbee River in Sumter County (Figs. 1 and 11) is much

more complex than the other sections in the area as indicated by almost all previous workers (e.g., Smit et al., 1996). The succession has been described by a number of geoscientists, with many different interpretations of-fered by way of explanation (Jones, 1967; Copeland et al., 1976; Mancini et al., 1989; Mancini and Tew, 1990; Smit et al., 1996; Smith, 1997; Wawak, 2007). One significant difference at this location is the presence of faults, some of which appear to displace only the Cretaceous strata while others also affect the overlying Paleo-cene succession.

At this locality, the PBC is a light-colored, blue-grey, intensely bioturbated calcareous mudstone (often de-scribed as a chalk). Compared to the PBC of the Braggs and Mussel Creek sections, the sediments at Moscow Landing have a higher carbonate content and are, therefore, much harder in the field. Approximately 1–3 m be-low the top of the PBC is a quite distinctive layer of re-worked, abraded macrofossils (mainly large Exogyra spp., Pycnodonte spp., and Pterotrigonia spp.). This layer is also quartz-rich, with visible shark’s teeth and small phosphate clasts. Along much of the river bank outcrop, the shell bed is sub-parallel to the top-PBC surface, but in one area there appears to be some convoluted bedding and the shell layer is up-turned, as though folded. This was also recorded by Smit et al. (1996, their figs. 3 and 4). The disruption to the shell bed (and the uppermost PBC) was not indicated by Mancini et al. (1989, their fig. 2) or several other workers. The presence of a lower, additional shell bed, parallel to that described above has also been missed.

Figure 6. General view of the Mussel Creek section viewed from the east bank of the creek. The sec-tion is almost 100% exposed along its ~100 m length.

Hart et al.

Towards the southern end of the section, approaching a concrete structure on the riverbank, there is a ‘simple’ situation with the shell bed, within the bioturbated PBC, overlain by a prominent, bioturbated horizon that is immediately overlain by gray mudstone, three nodular carbonate mudstone beds (Clayton Formation) and then a poorly exposed succession of mudstones and thin carbonate beds which represent the overlying Porter’s Creek Formation (Fig. 12). To the north, in an area where the uppermost PBC (and shell bed) are disrupted, a brown sandstone bed (with rare spherules) forms an intervening package of sediment with intense bioturbation of the top-PBC interface (Fig. 13). This brown sandstone is intermittently present, but then forms a continuous lay-er before expanding northwards into a ‘channel-like’ structure (Fig. 14) where there is a significant thickness of coarse brown sandstone, abundant large clasts of PBC, abundant re-worked Cretaceous macrofossils and micro-fossils. This unit also contains significant concentrations of rounded PBC clasts, cross-bedded sands and shell debris accumulations (Fig. 15). The northern edge of this structure is terminated by a fault (or faults) and, in a small cliff upstream of this ‘channel’ there is a return to the top-PBC, brown sandstone with burrows, small Nep-tunian dykes, and the overlying gray mudstones and carbonate-rich beds (Fig. 16). Along the entire section to the south of the site of the old bridge there appears to be three of these channel-like features, all of which appear to be associated with faulting.

Figure 7. The Pine Barren Member (Clayton Formation) along the dirt road approaching the bridge over Mussel Creek. The limestone and mudstone succession appears to exhibit a Milanković cyclicity. This succession of the Pine Barren Member is comparable to that described at Braggs by Donovan et al. (1988) and Miller’s Ferry by Olsson et al. (1996, their figs. 3, 4, 5B, 6A, 6C, and 8A).

INTERPRETATION Using the K/Pg boundary definition of the International Commission on Stratigraphy (Molina et al., 2006),

we have been able to piece together the sequence of events in all these sections and linked our preliminary inter-pretations to both the now-buried Miller’s Ferry sections (Olsson et al., 1992) and the Brazos River area (Hart et al., 2012). The top-PBC, which Donovan et al. (1988) have shown to be a regional unconformity, marks the K/Pg boundary and—at most locations—is a smooth but undulating, bioturbated surface. Savrda (1993, his fig. 1) denoted this as ‘surface A’ and indicated that the erosion of this surface truncated burrows, infaunal bivalves and other structures. This surface can be seen in the correlation diagrams of several other workers, including Mancini et al. (1989, their fig. 2), Olsson et al. (1996, their fig. 11) and Smit et al. (1996, their fig. 24). In-filling this un-dulating sea-floor topography is a variety of sediments, mainly sands, with reworked clasts of PBC, Cretaceous fossils and other debris. The Moscow Landing, Shell Creek, and Miller’s Ferry successions contain spherules

Figure 8. The Cretaceous/Paleogene boundary in Mussel Creek. This illustration of the top-PBCF surface is located near the pick in Figure 5. In this location, the Clayton Basal Sand is a yellow-colored mudstone with a reduced sand content: Compare to Figures 9 and 10. The overlying, basal, Pine Bar-ren Member calcarenite bed is seen in the upper part of the photograph. There is no evidence of signif-icant bioturbation at the interface, unlike the top-PBC surface. One large burrow-system is visible near the ruler which has a scale in inches and centimeters.

within these sediments (in variable densities), confirming that the top-PBC surface is the K/Pg event and that Chicxulub ejecta is being re-worked into subsequent sediments. None of these sections record a graded ejecta-fall layer as described in places such as Demerara Rise (MacLeod et al., 2007). Overlying this series of deposits is another prominent erosion surface (sometimes with bioturbation) that can be traced through all the Alabama locations, although the intensity of bioturbation indicated by Smit et al. (1996, their fig. 24) at Moscow Landing has not been observed (e.g., Braggs, Mussel Creek) in all locations. This surface at the base of the Pine Barren Member has been shown to approximate the base of P1a (Olsson and Liu, 1993; Olsson et al., 1996, their fig. 11) and our data confirm this determination. At all locations other than Moscow Landing, the smoothed, undulating top-PBC surface does not appear to represent localized channel features, and the extensive nature of the surface has been demonstrated by Donovan et al. (1988, their fig. 3). If this surface was generated by the tsunami from the Chicxulub impact site, as was the case in the Brazos River area of Texas (Hart et al., 2012), then the wave(s) removed all the unconsolidated muds and scoured down into the consolidated PBC, providing all the re-worked clasts seen in the overlying Clayton Basal Sands. The presence of re-worked spherules, above the top-PBC For-mation surface, confirms the sequence of events is comparable to that described in Hart et al. (2012) and Yancey and Liu (2013). The presence of graded, cross-bedded, sandstones and mudstones (with plant debris) in the over-lying Clayton Basal Sands are not comparable to the clast-bearing mudstones of the Brazos River area that have been interpreted as mass flow deposits (Yancey and Liu, 2013). These authors have, again, raised the question as to how much seismic ‘shaking’ affected the Gulf Coast area at the time of the Chicxulub impact. Recently Moore

Hart et al.

Figure 9. The smooth, but undulating, top-PBC surface in Mussel Creek is overlain by the Clayton Basal Sands which are yellow-colored cross-bedded sandstones and mudstones that contain exotic blocks (near the ruler and pick) and abundant fragments of charcoal. The overlying prominent ledge is formed by the calcarenite at the base of the Pine Barren Member.

and Sharma (2013) have indicated that the impact, by a long-period comet, may have been much smaller than earlier suggested for the bolide (Hildebrand et al., 1991), with less ‘rocky’ material. To create the size of crater seen on the Yucatan Peninsula, a smaller object would have been travelling faster, and probably created the same level of seismic shock.

If the mid-Alabama area was affected by seismic shaking, then all the disturbed material was removed by the tsunami—as it was in Texas—together with the unconsolidated sea floor sediments. The re-worked PBC clasts are all consolidated and must have been exhumed by the wave(s). The smooth surface of the top-PBC Formation must have been eroded down to consolidated sediments as burrows at, or below, the surface have not suffered subsequent compaction which would have been the case had the sediments been un-consolidated.

The complex situation in the Moscow Landing succession, with the up-turned PBC Formation, appears to be a local response to the faulting. As the ‘channel-like’ structures are terminated by faulting, their true geometry cannot be determined and more, detailed, mapping is required to interpret this section.

SUMMARY Using the revised definition for the K/Pg boundary (Molina et al., 2006) and information from recent field-

work in the Brazos River area of Texas, it has been possible to further reflect on the well-known K/Pg boundary

Figure 10. Close-up of a large, angular, clast of consolidated PBC within the Clayton Basal Sands just above the top-PBC surface. The finely laminated, heterolithic mudstones and sandstones of the Clay-ton Basal Sands are visible. Finely disseminated charcoal is visible, as well as some larger plant frag-ments. The ruler is in inches and centimeters.

Hart et al.

successions in Alabama. There is widespread evidence of end-Cretaceous erosion of the pre-existing in-situ suc-cession of the Prairie Bluff Chalk, with deposition of the Clayton Basal Sands in-filling an undulating topography on the sea floor with locally derived sands, PBC clasts, reworked fossils and re-worked Chicxulub spherules. There is no evidence of exposure, sub-aerial erosion, or karstification, with almost all surfaces showing active bioturbation by marine infauna.

ACKNOWLEDGMENTS The authors wish to thank their respective institutions for support in the field. The Annie Greenly Fund of

the Geological Society of London supported some of the travel costs of M. B. Hart. Mr John Abraham has assist-ed M. B. Hart with some of the figures used in this paper. The authors wish to thank many of their colleagues for helpful discussions of the K/Pg boundary problem. Prof. R. Olsson (Rutgers University) is particularly thanked for his sharing of information from the Miller’s Ferry construction site: a locality which is no longer available for field study.

Figure 12. The southern part of the outcrop at Moscow Landing showing the Prairie Bluff Chalk For-mation overlain by three mudstone-limestone couplets (Pine Barren Member, Clayton Formation) and the more calcareous mudstone succession of the Porter’s Creek Formation (partly obscured by vegeta-tion). The highly bioturbated top-PBC surface is seen in the center of the photograph.

REFERENCES CITED

Alegret, L., E. Thomas, and K. C. Lohman, 2011, End-Cretaceous marine mass extinction not caused by productivity collapse: Proceedings of the National Academy of Science, doi:10.1073/pnas.1110601109/-/DCSupplemental.

Baum, G. R., and P. R. Vail, 1988, Sequence stratigraphic concepts applied to Paleogene outcrops, Gulf and Atlantic

basins, in C. K. Wilgus, B. S. Hastings, H. Posmaentier, J. Van Wagoner, C. A. Ross, and C. G. St. C. Kendall, eds., Sea level changes—An integrated approach: Society of Economic Paleontologists and Mineralogists Special Publication 42, Tulsa, Oklahoma, p. 309–327.

Bralower, T. J., C. K. Paull, and R. M. Leckie, 1998, The Cretaceous-Tertiary boundary cocktail: Chicxulub impact

triggers margin collapse and extensive gravity flows: Geology, v. 26, p. 331–334. Cepek, P., W. W. Hay, B. A. Masters, and T. R. Worsley, 1968, Calcareous plankton in samples from field trip stops, in

Facies changes in the Selma Group in central and eastern Alabama: Alabama Geological Society 6th Annual Field Trip Guidebook, Tuscaloosa, p. 33–40.

Copeland, C. W., J. G. Newton, et al., 1976, Cretaceous and Tertiary faults in southwestern Alabama: Alabama Geo-

logical Society 14th Annual Field Trip Guidebook, Tuscaloosa.

Hart et al.

Figure 13. View of the highly bioturbated contact between the top-PBC surface and the Clayton Basal Sand (near foreground). The shell concentration within the uppermost PBC is seen just above the rul-er, which has scales in both inches and centimeters. Some of the sub-horizontal, branching burrows are ~75 cm in length and ~5 cm in diameter.

Copeland, C. W., and E. A. Mancini, 1986, Cretaceous-Tertiary boundary southeast of Braggs, Lowndes County, Ala-

bama, in T. L. Neathery, ed., Geological Society of America Centennial Field Guide, v. 6—Southeastern section: Geological Society of America, Boulder, Colorado, p. 369–372.

Donovan, A. D., 1986, Sedimentology of the Providence Formation, in J. Reinhardt, ed., Stratigraphy and sedimentolo-

gy of continental, near-shore, and marine Cretaceous sediments of the Eastern Gulf Coastal Plain: American Asso-ciation of Petroleum Geologists / Society of Economic Paleontologists and Mineralogists Field Trip Guidebook 3, Tulsa, Oklahoma, p. 29–44.

Donovan, A. D., and P. R. Vail, 1986, Sequence stratigraphy of the K–T boundary in Alabama: A noncatastrophic

alternative: Geological Society of America Abstracts with Programs, v. 18, p. 587. Donovan, A. D., G. R. Baum, G. L. Blechschmidt, T. S. Loutit, C. E. Pflum, and P. R. Vail, 1988, Sequence

stratigraphic setting of the Cretaceous-Tertiary boundary in central Alabama, in C. K. Wilgus, B. S. Hastings, H. Posmaentier, J. Van Wagoner, C. A. Ross, and C. G. St. C. Kendall, eds., Sea level changes—An integrated approach: Society of Economic Paleontologists and Mineralogists Special Publication 42, Tulsa, Oklahoma, p. 299–308.

Figure 14. Panoramic view of one of the ‘channel-like’ fault-controlled structures in the center of the Moscow Landing outcrop. The PBC forms the foreground, overlain by a complex series of sedimen-tary features that involve the spherule-bearing, brown sandstones of the Clayton Basal Sands. In the center of the image, near the hammer, is a sandstone bed with large PBC clasts, overlain by an accu-mulation of PBC clasts in a pale, carbonate-rich matrix. This is overlain by a prominent thickness of brown sandstones, a detail of which is shown in Figure 15. The thickness of brown, conglomeratic sandstone increases to the right of the photograph before being terminated by a fault (or faults). The brown sandstones of the Clayton Basal Sands expand, contract and even bifurcate in a most complex pattern and include, in places (center-left of photograph) very large blocks of pale grey PBC.

Hart et al.

Habib, D., S. Moshkovitz, and C. Kramer, 1992, Dinoflagellate and calcareous nannofossil response to sea-level change

in Cretaceous-Tertiary boundary sections: Geology, v. 20, p. 165–168. Haq, B. U., J. Hardenbol, and P. R. Vail, 1987, Chronology of fluctuating sea levels since the Triassic: Geology, v. 20,

p. 165–168. Haq, B. U., J. Hardenbol, and P. R. Vail, 1988, Mesozoic and Cenozoic chronostratigraphy and cycles of sea-level

change, in C. K. Wilgus, B. S. Hastings, H. Posmaentier, J. Van Wagoner, C. A. Ross, and C. G. St. C. Kendall, eds., Sea level changes—An integrated approach: Society of Economic Paleontologists and Mineralogists Special Publication 42, Tulsa, Oklahoma, p. 72–108.

Hart, M. B., S. E. Feist, G. D. Price, and M. J. Leng, 2004, Reappraisal of the K–T boundary succession at Stevns Klint,

Denmark: Journal of the Geological Society, London, v. 161, p. 885–892. Hart, M. B., S. E. Feist, E. Håkansson, C. Heinberg, G. D. Price, M. J. Leng, and M. P. Watkinson, 2005, The Creta-

ceous-Paleogene boundary succession at Stevns Klint, Denmark: Foraminifers and stable isotope stratigraphy: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 224, p. 6–26.

Figure 15. Part of the Clayton Basal Sands associated with the channel-like structure shown in Figure 14. The lower part (near the ruler with both inch and centimeter scales) contains rounded clasts of re-worked PBC. This is overlain by some plane beds, with rare, small, PBC clasts, and—with a sharp boundary—a part of the Clayton Basal Sands with abundant broken, re-worked macrofossils (mainly bivalves). This unit grades upwards into a series of sandstones with a more ‘hummocky’ style of sedi-mentation.

Hart, M. B., T. E. Yancey, A. D. Leighton, B. Miller, C. Liu, C. W. Smart, and R. J. Twitchett, 2012, The Cretaceous-

Paleogene boundary on the Brazos River, Texas: New stratigraphic sections and revised interpretations: Gulf Coast Association of Geological Societies Journal, v. 1, p. 69–80.

Hildebrand, A. R., G. T. Penfield, D. A. Kring, M. Pilkington, and Z. A. Camargo, 1991, Chicxulub crater: A possible

Cretaceous/Tertiary boundary impact crater in the Yucatan Peninsula: Geology, v. 19, p. 232–255. Jones, D. E., 1967, Geology of the Coastal Plain of Alabama: Geological Society of America Field Trip Guidebook 1,

80th Annual Meeting, Boulder, Colorado, 113 p. Jones, D. S., P. A. Mueller, J. R. Bryan, J. P. Dobson, J. E. Channell, J. C. Zachos, and M. A. Arthur, 1987, Biotic,

geochemical, and paleomagnetic changes across the Cretaceous-Tertiary boundary at Braggs, Alabama: Geology, v. 15, p. 311–315.

King, D. T., Jr., and L. W. Petruny, 2003, Cretaceous-Tertiary distal impact ejecta in Alabama—Shell Creek strati-

graphic section: Geological Society Annual Meeting, Seattle, Washington, Abstract #26–9, .

Figure 16. The lower 60% of this photograph shows ‘disturbed’ PBC, which appears to have been bro-ken up and (possibly) transported in some form of mass flow deposit. There is no base to this disturbed material as it grades downwards into in situ calcareous mudstones. There is, therefore, no apparent surface on which movement has occurred. Near the ruler (with inch and centimeter scales) there are wedge-shaped ‘dykes’ of brown sandstone (Clayton Basal Sands) let down into the PBC and, associated with this horizon, is a prominent Fe-stain. This surface appears to be that illustrated by Smit et al. (1996, their fig. 3, surface E).

King, D. T., Jr., and L. W. Petruny, 2004, Cretaceous-Tertiary boundary microtektite-bearing sands and tsunamic beds, Alabama Gulf Coastal Plain: Lunar and Planetary Science Conference Abstract 1804.

King, D. T., Jr., and L. W. Petruny, 2005, Cretaceous-Tertiary boundary microtektite-bearing sands and tsunamic beds,

Alabama Gulf Coastal Plain: Gulf Coast Association of Geological Societies Transactions, v. 55, p. 382–391. LaMoreaux, P. E., and I. D. Toulmin, 1959, Geology and groundwater resources of Wilcox County, Alabama: U.S.

Geological Survey and Geological Survey of Alabama County Report 4, Tuscaloosa, 280 p. MacLeod, K. G., D. L. Whitney, B. T. Huber, and C. Koeberl, 2007, Impact and extinction in remarkably complete

Cretaceous-Tertiary boundary sections from Demerara rise, tropical western North Atlantic: Geological Society of America, Bulletin, v. 119, p. 101–115.

Mancini, E. A., and B. H. Tew, 1990, Tertiary sequence stratigraphy and biostratigraphy of southwestern Alabama:

Geological Society of America Southeastern Section Field Trip Guidebook 1, 39th Annual Meeting, Tuscaloosa, Alabama, 51 p.

Mancini, E. A., and B. H. Tew, 1990, Eustasy versus subsidence: Lower Paleocene depositional sequences from south-

ern Alabama, eastern Gulf Coastal Plain: Geological Society of America Bulletin, v. 105, p. 3–17. Mancini, E. A., B. H. Tew, and C. C. Smith, 1989, Cretaceous-Tertiary contact, Mississippi and Alabama: Journal of

Foraminiferal Research, v. 19, p. 93–104. Molina, E., L. Alegret, I. Arenillas, J. A. Arz, N. Gallala, J. Hardenbol, K. von Salis, E. Steurbaut, N. Vandenberghe,

and D. Zaghbib-Turki, 2006, The global boundary stratotype section and point for the base of the Danian Stage, Paleocene, Paleogene, “Tertiary”, (Cenozoic) at El Kef, Tunisia—Original definition and revision: Episodes, v. 29, p. 263–278.

Moore, J. R., and M. Sharma, 2013, The K–Pg impactor was likely a high velocity comet: Lunar and Planetary Science

Conference Abstract 2431. Olsson, R. K., and C. Liu, 1993, Controversies on the placement of Cretaceous-Paleogene boundary and the K/P mass

extinction of planktonic foraminifera: Palaios, v. 7, p. 127–139. Olsson, R. K., C. Liu, M.-P. Aubry, and T. Ehrendorfer, 1992, A biostratigraphically continuous Cretaceous/Paleogene

section at Millers Ferry, Alabama: Support for a bolide impact: Geological Society of America Abstracts with Programs.

Olsson, R.K., C. Liu, and M. van Fossen, 1996, The Cretaceous-Tertiary catastrophic event at Miller’s Ferry, Alabama,

in G. Ryder, D. Fastovsky, and S. Gartner, eds., The Cretaceous-Tertiary boundary event and other catastrophes in earth history: Geological Society of America Special Paper 307, Boulder, Colorado, p. 263–277.

Savrda, C. E., 1993, Ichnosedimentologic evidence for a noncatastrophic origin of Cretaceous-Tertiary boundary sands

in Alabama: Geology, v. 21, p. 1075–1078. Smit, J., T. B. Roep, W. Alvarez, A. Montanari, P. Claeys, J. M. Grajales-Nishimura, and J. Bermudez, 1996, Coarse-

grained, clastic sandstone complex at the K/T boundary around the Gulf of Mexico: Deposition by tsunami waves induced by the Chicxulub impact?, in G. Ryder, D. Fastovsky, and S. Gartner, eds., The Cretaceous-Tertiary boundary event and other catastrophes in earth history: Geological Society of America Special Paper 307, Boul-der, Colorado, p. 151–182.

Smith, C. C., 1997, The Cretaceous-Tertiary boundary at Moscow Landing, west-central Alabama: Gulf Coast Associ-

ation of Geological Societies Transactions, v. 47, p. 533–540. Smith, E. A., L. C. Johnson, and D. W. Langdon, 1984, Report on the geology of the Coastal Plain of Alabama, with

contributions to its paleontology by T. H. Aldrich and K.M. Cunnigham: Geological Survey of Alabama Special Report 6, Tuscaloosa, 759 p.

Thierstein, H. R., 1981, Late Cretaceous nannoplankton and the change at the Cretaceous-Tertiary boundary, in J. E.

Warme, R. G. Douglas, and E. L. Winterer, eds., The Deep Sea Drilling Project: A decade of progress: Society of Economic Paleontologists and Mineralogists Special Publication 32, Tuscaloosa, p. 355–394.

Hart et al.

Van Wagoner, J. C., H. W. Posamentier, R. M. Mitchum Jr., P. R. Vail, J. F. Sarg, T. S. Loutit, and J. Hardenbol, 1988, An overview of the fundamentals of sequence stratigraphy and key definitions, in C. K. Wilgus, B. S. Hastings, H. Posamentier, J. Van Wagoner, C. A. Ross, and C. G. St. C. Kendall, eds., Sea level changes—An integrated approach: Society of Economic Paleontologists and Mineralogists Special Publication 42, Tulsa, Oklahoma, p. 39–45.

Wawak, B. E., 2007, K/T boundary and Paleogene stratigraphy of southwestern Alabama: Lafayette Geological Socie-

ty Field Trip Guidebook, Louisiana, 97 p. Worsley, T., 1974, The Cretaceous-Tertiary boundary event in the ocean, in W. W. Hay, ed., Studies in paleo-

oceanography: Society of Economic Paleontologists and Mineralogists Special Publication 20, Tulsa, Oklahoma, p. 94–125.

Yancey, T. E., and C. Liu, 2013, Impact-induced sediment deposition on an offshore , mud-substrate continental shelf,

Cretaceous-Paleogene boundary, Brazos River, Texas, U.S.A.: Journal of Sedimentary Research, v. 83, p. 354–367.

Zachos, J. C., M. A. Arthur, and W. E. Dean, 1989, Geochemical and paleoenvironmental variations across the Creta-

ceous/Tertiary boundary at Braggs, Alabama: Palaeogeography, Palaeclimatology, Palaeoecology, v. 69, p. 245–266.

the cretaceous/paleogene boundary events in the gulf coast

Documents

agu, san francisco, 2013 modelled insights into climate...

geochemical variation of the late cretaceous …ˆ’41−...

further paleogene and cretaceous sediment cores from the...

21. cretaceous and paleogene calcisphaerulidae from … ·...

on impact and volcanism across the cretaceous-paleogene...

cretaceous-paleogene santa elena block assessment unit...

a symposium focusing on cretaceous and paleogene

ordovician devonian permian triassic cretaceous ... ·...

the cretaceous–paleogene (k–p) boundary at brazos,...

microbiostratigraphy, microfacies and sequence stratigraphy...

deep-sea environments across the cretaceous/paleogene...

late cretaceous to early paleogene foraminiferal biozones...

latest cretaceous–earliest paleogene vegetation and...

macrofossil evidence for a rapid and severe cretaceous...

palaeogeography, palaeoclimatology, palaeoecology · 2019....

mass extinction of birds at the cretaceous paleogene (k pg)...

chicxulub impact event is cretaceous/paleogene boundary in...

what is the role of palaeogeography across the cretaceous...

on transient climate change at the cretaceous−paleogene...

potential cretaceous-paleogene boundary tsunami deposit in...