Miocene Marine Diatoms From the Choptank Formation, Calvert County, Maryland GEOLOGICAL SURVEY PROFESSIONAL PAPER 910 Miocene Marine Diatoms From the Choptank Formation, Calvert County, Maryland By GEORGE W. ANDREWS GEOLOGICAL SURVEY PROFESSIONAL PAPER 910 A description of a 1niddle hi iocene marine diatonl assentblage for the Atlantic Coastal Plain region and its stratigraphic and paleoecologic significance UNITED STATES GOVERN"IENT PRI~ TI~G OFFICE, '\\rASHINGTON 1976 UNITED STATES DEPARTMENT OF THE INTERIOR THOIVIAS S. KLEPPE, Secretary GEOLOGICAL SURVEY V. E. l\icKelvey, Director Library of Congress Cataloging in Publication Data Andrews, George William, 1929- Miocene marine diatoms from the Choptank Formation, Calvert County, Maryland. (Geological Survey professional paper ; 910) Bibliography: p. Includes index. Supt. of Docs. no.: I 19.16:910 1. Diatoms, Fossil. 2. Paleontology-Miocene. 3. Paleontology-Maryland-Calvert Co. I Title II. Series: United States. Geological Survey. Professional paper ; 910. QE955.A62 561'.93 74-34195 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-02735-6 CONTENTS Page Abstract ----------------------------------------------------------------- 1 Introduction -------------------------------------------------------------- 1 Acknowledgments --------------------------------------------------------- 1 Miocene stratigraphy ____________________________________ - ----------------- 2 Calvert Cliffs locality _ ___ _ _ __ _ ___ _ __ ____ _ __ __ _ __ _ __ __ _ __ _____ _ ___ _ ____ ____ 2 Previous work on diatoms of the Chesapeake Group -------------------------- 4 Diatom assemblage ------------------------------------------------------- 6 Systematic descriptions ________________________________ -------------------- 7 Selected references -------------------------------------------------------- 23 Index ------------------------------------------------------------------- 25 ILLUSTRATIONS [Plates follow index I PLATE 1. Melosira, Paralia, Pyxidicula, and Stephanopy;r;is. 2. Stephanopyxis and Coscinodiscus. 3. Coscinodiscns, Craspedodiscus, and Actinocyclus. 4. H yalodiscus and Actinoptyclws. 5. Rattrayella, Liradiscus, Biddulphia, Goniothecium, Encampia, Terpsinoe, Trice'ratium, and Periptera. 6. Xanthiopyxis, Dossctia, Stephanogonia, Psemlopyxilla, and Rhaphoneis. 7. Rhaphoncis, Thalassioncma, Tlwlassiothrix, Grammatophora, Cocconcis, Na'vicula,, Diploneis, Pleurosigma, and Amphora. Page FIGURE 1. Index map showing location of the Calvert Cliffs area in Maryland --------------------------------- 3 2. Map showing location of the Choptank fossil diatom locality, Calvert County, Md -------------------- 4 TABLE Page TABLE 1. Comparison of diatom taxa from the Chesapeake Group. ___________________________________________ _ 7 III MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION, CALVERT COUNTY, MARYLAND By GEORGE W. ANDRE\VS ABSTRACT been found in the St. Marys Formation of Mary­ The Choptank Formation of Miocene age contains diatom­ land. The upper shell bed of the Choptank Forma­ bearing strata in outcrops in the Calvert Cliffs on the west tion, exposed in the Calvert Cliffs southeast of Long side of Chesapeake Bay. A good diatom assemblage is pres­ ent in the upper part (stratigraphic zone 19 of Shattuck Beach, Calvert County, Mel., contains a diatom as­ [1904] ) in the section exposed immediately north of the Cal­ semblage of 60 taxa; this assemblage is the specific vert Cliffs nuclear power plant, Calvert County, Md. subject of this report. The assemblage is distinct­ The Choptank Formation has long been correlated with the ly more modern in aspect than those reported previ­ middle Miocene of the European section principally on the ously from the Calvert Formation, and it is con­ basis of its molluscan fauna. Recent studies of planktonic Foraminifera suggest that it correlates with either the upper sidered to be a representative middle Miocene dia­ Langhian, the Serravalian, or the Tortonian Stages (middle tom assemblage developed in a shallow marine shelf to upper Miocene) , but a more precise determination has not environment. It occurs in a formation of fairly well yet been made. The macrofauna and microfauna of the forma­ determined stratigraphic position (specifically, the tion are suggestive of deposition in a shallow, inner-shelf base of lithologie unit 18 of Gernant [ 1970, p. 7 4 ] marine environment. The diatom assemblage in the upper part of the Choptank Formation reinforces these conclusions. from stratigraphic zone 19 of Shattuck [ 1904 ] of There are fewer extinct taxa and more still-living taxa in the the Choptank Formation) and hence should be ade­ Choptank assemblage than in the older assemblages present quate to serve the purpose of this study-to provide in the underlying Calvert Formation. The Choptank assem­ data on the occurrence of fossil marine diatoms blage consists of exclusively marine diatom species and is from a stratum of known geologic age and to n1ake dominated by the more robust centric planktonic diatoms. The presence of these diatoms is indicative of a shallow, agitated these data available for regional geologie correla­ depositional environment and the fact that only a low per­ tion and elating. centage of benthonic species is present suggests muddy bot­ tom conditions and a shifting substratum. i\CKNOWLEDG i\IENTS Sixty taxa of marine diatoms have been identified in this Choptank 'assemblage; seven are previously unreported, 18 are Access to the collection locality was made possible extinct forms known only from Miocene rocks, six are ex­ through the kind cooperation of Robert E. Ger­ tinct forms ranging outside Miocene rocks, and 29 are long­ nant, University of Wisconsin-Milwaukee and Lin­ ranging forms still living in modern oceans. This study of the coln Dryden, Bryn Mawr College, who were in Choptank diatoms adds to our knowledge of the stratigraphic charge of the Calvert Cliffs Project sponsored by ranges of diatom species and thus increases their usefulness in stratigraphic correlation and geologic age determination the Maryland Academy of Sciences. John L. Stone, of Tertiary strata. Washington, D.C., prepared several slides for the writer with Naphrax, 1 a high-index mounting me­ INTRODUCTION diunl; in addition, he searched a number of slides This report is the first part of a study of fossil for additional specimens and provided helpful dis­ marine diatoms in the Tertiary strata of the Atlan­ cussion of the diatom identification and taxonomy. tic Coastal Plain. The Calvert, Choptank, and St. Kenneth E. Lohman, Division of Paleobotany, The Marys Formations, all part of the Chesapeake Smithsonian Institution, Blake Blackwelder, Joseph Group, crop out as a sequence of clastic deposits E. Hazel, and Sergius H. Mamay, U.S. Geological along the Atlantic Coastal Plain of Maryland and Survey, have reviewed the manuscript and their Virginia. The Calvert Formation is highly to mod­ many helpful comments are appreciated. erately diatomaceous, the Choptank Formation is 1 Anv use of tJ·a(le names in this publication i,; for descriptive purposes sparingly diatomaceous, and diatoms have not yet only a'Od does not constitute endorsement by the U.S. Geological Survey. 1 2 MIOCENE MARINE DIATOMS FROM THE CHOPTANK FORMATION lVIIOCENE STRATIGRAPHY ing of the shells and secondary carbonate deposi­ The Miocene strata of the Middle Atlantic States tion. The Fairhaven Diatomaceous Earth Member of have been well known since early colonial times be­ the Calvert Formation rests unconformably on un­ cause of their excellent exposure along Chesapeake derlying Eocene strata. It may be readily distin­ Bay in Maryland and along the Potomac River es­ guished from the Eocene deposits, which contain tuary in Virginia. Shattuck (1904, p. 41-64) gives abundant glauconite and quartz granules but lack an exhaustive bibliography of publications on the diatoms. The Fairhaven consists mainly of an im­ Miocene strata of Maryland for the years 1669- pure silty diatomite. The overlying Plum Point Marl 1903. Modern stratigraphic terminology began with Member of the Calvert Formation consists of a Darton (1891, p. 433), who named the entire Mio­ series of impure fine sands, silts, and clays and beds cene stratigraphic section of the region the Chesa­ composed primarily of mollusk shells in a sand-silt­ peake Formation. This name was modified by Dall clay matrix. This part of the Calvert Formation is and Harris ( 1892, p. 123) to Chesapeake Group and moderately diatomaceous. was extended to include all Miocene strata of the The Choptank Formation is said by Shattuck Atlantic Coastal Plain. The Chesapeake Group in (1904, p. 80) to rest unconformably upon the Cal­ Maryland was first divided into the Calvert, Chop­ vert Formation. Although this may be apparent on tank, and St. Marys Formations by Shattuck (1902). a regional basis, the Choptank lithology is so similar The Chesapeake Group in Maryland was described to that of the upper part of the Calvert Formation in detail by ShaUuck ( 1904, p. 69-92) , and the sys­ that an exact determination of the contact is diffi­ tematic paleontology of various fossil groups of the cult in most outcrops. Two parts of the Choptank Miocene deposits of Maryland was also presented Formation, stratigraphic zones 17 and 19, are dis­ in that volume, including a chapter by C. S. Boyer tinguished primarily by their high content of mol­ (1904) on the diatoms. Shattuck (1904) divided the lusk shells. The