Biotic, geochemical, and paleomagnetic changes across the Cretaceous/Tertiary boundary at Braggs, Alabama Douglas S. Jones Florida State Museum, University of Florida, Gainesville, Florida 32611 Paul A. Mueller, Jonathan R. Bryan, Jon P. Dobson, James E. T. Channell Department of Geology, University of Florida, Gainesville, Florida 32611 James C. Zachos, Michael A. Arthur Graduate School of Oceanography, University of Rhode Island Narragansett, Rhode Island 02882 ABSTRACT Exposed near Braggs, Alabama, is one of the few well-studied, nearly continuous shallow-marine Cretaceous/Tertiary boundary sec- tions; it allows a glimpse of the biotic and environmental changes that occurred in the latest Cretaceous to earliest Paleocene. Paleomagnetic, strontium isotopic, and biostratigraphic data closely constrain the age of a series of lithologic, geochemical, and biotic variations and suggest that no more than 100-200 ka could be missing at the boundary. A mqjor reduction in macrofaunal diversity associated with lithofacies changes occurs prior to but within 300 ka of the nannofossil-defined boundary. Approximately 40% of the apparent faunal reduction is attributed to the "Lazarus effect." Faunal and floral assemblages, trends in carbon isotopic composition of benthic invertebrates, and lithologic characteristics indicate that a latest Maestrichtian regression culminated near the boundary (Chron C29R; Micula murus zone), significantly later than recent estimates. Water depths at this site re- mained shallow during the subsequent early Paleocene (zone NP1) transgression and did not reach depths equivalent to those of the late Maestrichtian until zone NP2. Relatively minor climatic changes across the boundary are suggested by a <4 °C cooling trend seen in the oxygen-isotope paleotemperatures. A high-resolution ^Sr/^Sr rec- ord from well-preserved macrofossil calcite shows a pattern of smooth variation and elevated values near the boundary; however, the early Paleocene "spike" of other workers was not found. INTRODUCTION The hypothesis of Alvarez et al. (1980) that biotic extinctions at the Cretaceous/Tertiary (K/T) boundary were engendered by the impact of an extraterrestrial body has spawned considerable debate in the geological literature (e.g., McCartney and Nienstedt, 1986). As the controversy sur- rounding the K/T crisis expands, it promotes detailed examination of boundary sections worldwide. The most complete sections are found among pelagic sequences, whether on land or in the deep sea, and these continue to be investigated as the best record of K/T changes. Good shallow-marine sections, in contrast, have received less attention, probably because of reduced stratigraphic completeness associated with erosion or episodic sedimentation. Nonetheless, such sections warrant closer exami- nation in order to (1) determine how paleoceanographic changes in the pelagic realm relate to those in shallow-marine environments, (2) assess the rate and timing of extinctions among major invertebrate fossil groups, and (3) evaluate the paleoenvironmental information unique to these settings. We report here the results of a comprehensive investigation of a continuous shallow-marine K/T boundary sequence that crops out in the Gulf Coastal Plain of central Alabama. The K/T boundary is exposed in a road cut along the southwest side of State Highway 263, about 7.4 km southeast of the intersection of State Highways 21 and 263 at Braggs, Lowndes County, Alabama. This exposure is well known from previous investigations, which have been summarized by Copeland and Mancini (1986). Worsley (1974) considered it "the most nearly transitional section across the Cretaceous-Tertiary boundary in the world." Using available nannofossil biostratigraphic constraints, we developed a magnetostratig- raphy correlated to the geomagnetic time scale. Within this framework the patterns of macrofossil and geochemical (percent CaCC>3, S 18 0,6 13 C, 87 Sr/ 86 Sr) changes across the boundary were analyzed in detail. The results are illustrated in Figure 1 and discussed below. STRATIGRAPHY The Upper Cretaceous Prairie Bluff Chalk is about 30 m thick in western Lowndes County where it unconformably overlies the fossiliferous Ripley Formation. Exposed in the boundary section studies here are the top 2 m, which consist of grayish-black, silty to sandy, glauconitic, cal- careous clay that contains abundant microfossils and macrofossils. Overly- ing the Prairie Bluff Chalk is the lower or Pine Barren Member of the Paleocene Clayton Formation. This lower member is also ~30 m thick in this vicinity and is in turn overlain by the upper or McBryde Limestone Member of the Clayton. The Pine Barren Member consists of interbedded, fossiliferous, glauconitic, calcareous sandstone, marl, and limestone (Fig. 1A). According to Copeland and Mancini (1986), the contact between the Prairie Bluff Chalk and the Pine Barren Member of the Clayton is within an indurated, glauconitic, sandy limestone bed (bed 3) containing phosphatic pebbles and abundant fossils. Cross-bedded quartz- ose sands in the uppermost several centimetres of bed 2 occur at a possible erosional unconformity near the top of the Prairie Bluff Chalk which is less prominent here than at other K/T boundary sequences of the eastern Gulf Coastal Plain. Biostratigraphy The ages of the Prairie Bluff Chalk and the Pine Barren Member of the Clayton Formation southeast of Braggs have been discussed in several papers, most of which were summarized by Copeland and Mancini (1986). Mollusca and benthic foraminifera from this section suggest a shallow to middle continental shelf depositional setting, and therefore it is not too surprising that stratigraphically important planktonic foraminifera are sparse or lacking. Neither Abathomphalus mayaroensis (latest Maes- trichtian) nor Globigerina eugubina (earliest Danian) are present (Cepek et al., 1968; Gibson et al., 1982). The calcareous nannoplankton, however, can be used to identify latest Cretaceous and earliest Tertiary sediments at Braggs (Fig. IB). GEOLOGY, v. 15, p. 311-315, April 1987 311 on July 13, 2015 geology.gsapubs.org Downloaded from