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CAROLINA GEOLOGICAL SOCIETY<br />
<strong>Guidebook</strong> for 1992 Annual Meeting<br />
Pages 57-58<br />
THE MCBEAN FORMATION AND ORANGEBURG DISTRICT BED — COOK MOUNTAIN AND GOSPORT<br />
EQUIVALENTS (MIDDLE EOCENE) IN THE COASTAL PLAIN OF SOUTH CAROLINA<br />
David T. Dockey III<br />
Mississippi Office Of Geology<br />
Paul G. Nystrom, Jr.<br />
South <strong>Carolina</strong> <strong>Geological</strong> Survey<br />
INTRODUCTION<br />
The Orangebury District bed is an informal stratigraphic<br />
unit introduced by Dockery and Nystrom (1990, 1992) for<br />
fossiliferous sands at Orangeburg, South <strong>Carolina</strong>, that have<br />
a diverse silicified fauna. This bed disconformably overlies<br />
carbonates of the McBean Formation and underlies nonfossiliferous<br />
sands of the Jackson Group. Silicified fossil invertebrates<br />
of the Orangebury District bed are diverse and occur<br />
in thin concentrations at di<strong>as</strong>tems and lenses. The molluscan<br />
fauna includes 169 species and subspecies. A little more than<br />
a third of these species are known only from Orangeburg; a<br />
little less than a third occur in both the Cook Mountain Formation<br />
and Gosport Sand of the Gulf Co<strong>as</strong>tal Plain; and the<br />
remainder occur in either Cook Mountain or older units or<br />
the Gosport or younger units. Notably absent from this fauna<br />
are two Cook Mountain guide fossils, the bivalve Cubitostrea<br />
sellaeformis and Pteropsella lapidosa. The later species<br />
occurs <strong>as</strong> internal molds in McBean carbonates<br />
underlying the Orangeburg District bed at Orangeburg, and<br />
the former occurs in the McBean at nearby localities. Notably<br />
present in this fauna is the common occurrence of a Gosport<br />
guide fossil, the bivalve Glyptoactis (Claibornicardia)<br />
alticostata. B<strong>as</strong>ed on the absence and presence of these species,<br />
the mollusks indicate the Orangeburg District bed to be<br />
of Gosport age.<br />
RECOGNITION OF GOSPORT EQUIVALENT<br />
UNITS IN THE ATLANTIC COASTAL PLAIN<br />
Gosport age units have been recognized in the Santee<br />
Limestone of South <strong>Carolina</strong>’s lower co<strong>as</strong>tal plain by Ward<br />
et al. (1979) and in the C<strong>as</strong>tle Hayne Limestone of North<br />
<strong>Carolina</strong>’s co<strong>as</strong>tal plain by Zullo and Harris (1987). Ward et<br />
al. (1979) proposed the Martin Marietta Company Berkeley<br />
Quarry <strong>as</strong> the neostratotype of the Santee Limestone. Here<br />
they named two members, the Moultrie and Cross. The<br />
Moultrie is a moldic biosparite with abundant shells of Cubitostrea<br />
sellaeformis. This unit w<strong>as</strong> correlated with the middle<br />
Claiborne Lisbon Formation of Alabama. Disconformably<br />
overlying it are the bryozoan-brachiopod-bivalve biomicrites<br />
of the Cross Member. This member contains molds of a Gosport<br />
guide fossil, the bivalve Cr<strong>as</strong>satella alta, and w<strong>as</strong><br />
<strong>as</strong>signed a Gosport age. Ward et al. (1979) gave me<strong>as</strong>ured<br />
sections of the Cross Member at its type locality and at the<br />
Martin Marietta Company Georgetown Quarry and the Giant<br />
Portland Cement Company Quarry. Ward (personal communication)later<br />
found that the Cross type section contained<br />
Cubitostrea sellaeformis and decided that Giant Quarry<br />
would have been the better stratotype for the unit the authors<br />
originally had in mind. At the Giant Quarry, Gosport-age<br />
mollusks are present in an interval that lacks Cubitostrea<br />
sellaeformis. Thus, the Giant Quarry h<strong>as</strong> an unnamed carbonate<br />
sequence of Geosport age, while the type Cross is in<br />
the Cubitostrea sellaeformis zone of Cook Mountain age<br />
(Ward, personal communication).<br />
Zullo and Harris (1987) recognized four depositional<br />
sequences within the C<strong>as</strong>tle Hayne Limestone. Sequence 3 is<br />
equivalent to the Comfort Member of Ward et at. (1978) and<br />
w<strong>as</strong> correlated with the Cross Member of South <strong>Carolina</strong> and<br />
the Gosport Sand and lower Moodys Branch Formation of<br />
Alabama. The correlation with the lower Moodys Branch<br />
Formation w<strong>as</strong> b<strong>as</strong>ed on the occurrence of Periarchus lyelli.<br />
Another name for the “lower Moodys Branch Formation” is<br />
the Scutella (=Periarchus) bed, a unit that separates the Gosport<br />
Sand and calcareous “upper Moodys Branch Formation”<br />
in southern Alabama. This unit is characterized by an<br />
abundance of the flat echinoid Periarchus lyelli, a fossil<br />
incorrectly considered by some <strong>as</strong> a guide to the Moodys<br />
Branch Formation. Complete specimens of P. lyelli occur in<br />
the Gosport Sand at the cl<strong>as</strong>sic Little Stave Creek locality in<br />
Alabama. Zullo and Harris (1987) dismissed earlier published<br />
accounts of this echinoid in the Gosport Sand and considered<br />
the Scutella bed to be of Moodys Branch age. One<br />
occurrence they dismissed w<strong>as</strong> a citation by Cooke (1959, p.<br />
42 pl. 14, fig. 1-3) of P. lyelli from the Gosport Sand at<br />
Gopher Hill on the Tombigbee River in W<strong>as</strong>hington County,<br />
Alabama. At this locality (<strong>as</strong> observed by L.W. Ward and<br />
Dockery), the Scutella bed contains a typical Gosport molluscan<br />
fauna and is considered here to be a facies of the Gosport<br />
Formation. Therefore, Sequence 3 of Zullo and Harris<br />
(1987) is of Gosport age only.<br />
Dockery and Nystrom (1990, 1992) first recognized a<br />
Gosport age fauna in the upper co<strong>as</strong>tal plain of South <strong>Carolina</strong><br />
in what had cl<strong>as</strong>sically been called the McBean Formation.<br />
A construction site at Orangeburg showed a two-part<br />
division of the McBean consisting of an upper cl<strong>as</strong>tic<br />
sequence with a diverse silicified fauna containing Glyptoactis<br />
(Claibornicardia) alticostata and a lower carbonate<br />
57