Downloaded from fieldguides.gsapubs.org on October 16, 2015 OLD G The Geological Society of America Field Guide 40 2015 OPEN ACCESS Miocene stratigraphy and paleoenvironments of the Calvert Cliffs, Maryland Susan M. Kidwell Department of Geophysical Sciences, University of Chicago, 5734 S. Ellis Avenue, Chicago, Illinois 60637, USA David S. Powars Lucy E. Edwards 926A National Center, U.S. Geological Survey, Reston, Virginia 20192, USA Peter R. Vogt Marine Science Institute, University of California, Santa Barbara, California, USA ABSTRACT Miocene strata exposed in the Calvert Cliffs, along the western shore of the Ches- apeake Bay, Maryland, have a long history of study owing to their rich fossil record, including a series of spectacular shell and bone beds. Owing to increasingly refi ned biostratigraphic age control, these outcrops continue to serve as important references for geological and paleontological analyses. The canonical Calvert, Choptank, and St. Marys Formations, fi rst described by Shattuck (1904), are generally interpreted as shallowing-up, from a fully marine open shelf to a variety of marginal marine, coastal environments. More detailed paleoenvironmental interpretation is challeng- ing, however, owing to pervasive bioturbation, which largely obliterates diagnostic physical sedimentary structures and mixes grain populations; most lithologic con- tacts, including regional unconformities, are burrowed fi rmgrounds at the scale of a single outcrop. This fi eld trip will visit a series of classic localities in the Calvert Cliffs to discuss the use of sedimentologic, ichnologic, taphonomic, and faunal evidence to infer environments under these challenging conditions, which are common to Creta- ceous and Cenozoic strata throughout the U.S. Gulf and Atlantic Coastal Plains. We will examine all of Shattuck’s (1904) original lithologic “zones” within the Plum Point Member of the Calvert Formation, the Choptank Formation, and the Little Cove Point Member of the St. Marys Formation, as well as view the channelized “upland gravels” that are probably the estuarine and fl uvial equivalents of the marine upper Miocene Eastover Formation in Virginia. The physical stratigraphic discussion will focus on the most controversial intervals within the succession, namely the uncon- formities that defi ne the bases of the Choptank and St. Marys Formations, where misunderstanding would mislead historical analysis. Kidwell, S.M., Powars, D.S., Edwards, L.E., and Vogt, P.R., 2015, Miocene stratigraphy and paleoenvironments of the Calvert Cliffs, Maryland, in Brezinski, D.K., Halka, J.P., and Ortt, R.A., Jr., eds., Tripping from the Fall Line: Field Excursions for the GSA Annual Meeting, Baltimore, 2015: Geological Society of America Field Guide 40, p. 231–279, doi:10.1130/2015.0040(08). For permission to copy, contact [email protected]. © 2015 The Geological Society of America. All rights reserved. Gold Open Access: This chapter is published under the terms of the CC-BY license and is available open access on www.gsapubs.org. 231 Downloaded from fieldguides.gsapubs.org on October 16, 2015 232 Kidwell et al. INTRODUCTION analysis since the late nineteenth century (Darton, 1891; Har- ris, 1893; Shattuck, 1904). This attention owes largely to their The Calvert Cliffs are a relatively continuous series of natu- abundant and diverse shallow-marine faunas, especially mol- ral, 25–35 m- (80–120 foot-) high outcrops along the western lusks and vertebrates, which are concentrated in a series of spec- shore of the Chesapeake Bay in Calvert County, Maryland (Fig. tacular shell and bone beds within otherwise fi ne-grained and 1). The Miocene strata exposed in these cliffs—the canonical unlithifi ed siliciclastic sediments (Fig. 2). These resources have Calvert, Choptank, and St. Marys Formations (Shattuck, 1902, made these formations targets for testing general evolutionary 1904)—have been the focus of paleontologic and stratigraphic and paleoecological principles (e.g., Kelley, 1983, 1988; Dietl et Figure 1. Location of fi eld-trip stops in the Calvert Cliffs, along the Chesapeake Bay shoreline of Calvert County, Mary- land. Map adapted from Vokes (1957 [1968]). Downloaded from fieldguides.gsapubs.org on October 16, 2015 Miocene stratigraphy and paleoenvironments of the Calvert Cliffs, Maryland 233 Formation Member Surface Lithology Shattuck EC DiatomDinocyst Subseries Zone estuarine facies of post-St. Marys ? Eastover (?) Fm. pSM strata Windmill SHATTUCK 24 DN9 Point “WP-0” ~8.7 Ma DN8 upper none Little Cove SHATTUCK St. Marys St. Point SM-2 22-23 Figure 2. Generalized stratigraphic col- DN8 umn for Miocene strata in Maryland, STOP 9 SM-1 upper Miocene lower showing the increment exposed at each fi eld stop in the Calvert Cliffs. Shattuck- SM-0 SHATTUCK 21 Zones are informal lithologic units with Conoy distinct faunal content established by SHATTUCK 20 VII DN7 STOP 8 Shattuck (1904) and are still a practical Boston 11.6 Ma SHATTUCK 19 basis for sampling. Current defi nitions Cliffs STOP 7 CT-1 of formations are very similar to those of Shattuck (1904), member names as St. Leonard SHATTUCK 18 VI DN6 Choptank explained in the text; the informal “Gov- Drumcliff SHATTUCK 17 ernor Run” sand is an incised valley fi ll STOP 6 “Governor Run” CT-0 SHATTUCK 16B comprising the yellow-weathering up- SHATTUCK 16A dip sand within Shattuck’s original Zone 16 (herein, denoted as Shattuck-Zone DN5b SHATTUCK 15 V 16B) but not the blue-gray interbedded silty sand and clay that he recognized PP-3 SHATTUCK 14 downdip (herein, 16A), which Gernant (1970) erected as the Calvert Beach middle Miocene SHATTUCK 13 DN5a Member for outcrops there. Surfaces STOP 5 in Calvert Cliffs exposed strata are regionally signifi cant disconformi- ties, numbered successively within host STOP 4 Plum Point PP-2 SHATTUCK 12 STOP 3 lithostratigraphic units, starting with III-IV 0 to denote the basal unconformity SHATTUCK 11 III STOP 2 DN4 (FH—Fairhaven, Kidwell et al., 2015; PP—Plum Point, Kidwell, 1984; CT— SHATTUCK 10 PP-1 16.0 Ma Choptank, Kidwell, 1984; SM—St. SHATTUCK 9 Marys, Kidwell, 1997; pSM—post– SHATTUCK 8 St. Marys, Kidwell, 1997; “ WP-0”— Calvert SHATTUCK 7 inferred for base of Windmill Point SHATTUCK 6 Member, Kidwell et al., 2012). East SHATTUCK 5 Coast (EC) Diatom Zones are those of PP-0 SHATTUCK 4 Andrews (1978) and Abbott (1978). STOP 1 DN3 Dinocyst zones of de Verteuil and Nor- II ris (1996), refi ned by new samples and calibrated to the GTS-2012 (Grad- SHATTUCK 3B stein and Ogg, 2012) by Kidwell et al. (2012). Chart adapted from Kidwell et lower Miocene lower al. (2012). Fairhaven 10 m FH-3 not recognized DN2b,c “FH-2” none SHATTUCK 3A I SHATTUCK 2 DN2a FH-1 SHATTUCK 1 FH-0 none 23.0 Ma Nanjemoy Formation (part) EXPLANATION dense shell in sand+gravel fine sand diatomaceous bedded sandy mud muddy sand deposits mud+sand Downloaded from fieldguides.gsapubs.org on October 16, 2015 234 Kidwell et al. al., 2002; Buzas and Hayek, 2005; Grey et al., 2006), as well as fraction of the original skeletal input is likely to have survived, but for building systematic and phylogenetic understanding of major taphonomic analysis indicates that most skeletal assemblages are animal groups. Strata are also rich in biostratigraphically valu- time-averaged accumulations, refl ecting the mixing of many suc- able microfossils, most particularly diatoms and dinofl agellates cessive generations in the seabed before permanent burial (for inter- (Abbott, 1978; Andrews, 1978; de Verteuil and Norris, 1996). pretation of shell and bone beds, see Kidwell, 1991; Brett, 1995; These refi ned zonations make the Calvert Cliffs critical reference Rogers and Kidwell, 2007). Owing to bioturbation, the mixed-grain material for both the Salisbury embayment and broader Balti- populations of the siliciclastic matrix must also be viewed as time- more Canyon trough, and also support analyses of general rela- averaged records of grain supply and water energy. tionships between climate and sea-level change and the tectonic These gross lithologic features—largely massive, burrow- and geomorphic evolution of the Atlantic margin (e.g., Pazzaglia, mottled sediments, having mixed grain sizes and more or less 1993; Miller and Sugarman, 1995; Browning et al., 2006; Miller abundantly preserved skeletal remains—imply that each bed is et al., 2013). The cliffs have consequently been a perennial desti- a historically complex entity, and place a premium on paleoeco- nation for professional, student, and amateur fi eld trips. The most logical inference from trace and body fossils, including tapho- widely available fi eld guides, with measured sections for indi- nomic insights from modes of fossil preservation and concen- vidual localities, include Glaser (1968), Gernant (1970), Gernant tration. These challenging conditions are common in Cretaceous et al. (1971), Blackwelder and Ward (1976), Ward and Powars to Cenozoic marine strata throughout the U.S. Atlantic and Gulf (1989, 2004), Ward (1992), and Ward and Andrews (2008). Coastal Plains. The Miocene section in Maryland is widely accepted as a Here, we focus on the present state of understanding of shallowing-upward succession, from fully marine, open shelf to paleoenvironments in the Calvert Cliffs succession, reviewing the freshwater-infl uenced tidal coastal settings, based largely on fos- stratigraphy as a context for environmental history. We include sil content. It also includes signals of relatively high productivity, sedimentologic, ichnologic, taphonomic, and molluscan paleo- such as diatomaceous sediments (especially in the Calvert Forma- ecological evidence. For consistency with previous stratigraphic tion) and relatively large-bodied mollusks and vertebrates, espe- publications, and because molluscan paleoecological inference cially whales and sharks. However, two basic conditions chal- relies on functional groups (life habits and feeding modes) that lenge paleoenvironmental interpretation at the next level of detail. are generally conserved within families, we retain the classic tax- (1) Physical sedimentary structures are extremely rare. Most onomy of Clark et al. (1904). For updated molluscan systematics, beds are massive, either homogeneous or burrow-mottled.