Stratigraphic framework for geologic and geohydrologic studies of the subsurface Cretaceous section near Charleston, South Carolina

Abstract

This report illustrates the distribution of 11 formations that constitute the subsurface Cretaceous section near Charleston, South Carolina. Data from 14 drill holes are used to construct four geologic sections in southern Dorchester County and the Charleston-Mt. Pleasant area of central Charleston County (figs. 1, 2). Stratigraphic and lithologic information for the Cretaceous formations are discussed in the first part of this pamphlet along with brief descriptions of their ages and depositional paleoenvironments. Particular emphasis is placed on establishing criteria for recognizing the Cretaceous formations in cores and cuttings and on geophysical logs. The geohydrology of the Cretaceous section is discussed in the second part of this pamphlet. This latter section contains a discussion of the physical characteristics of Cretaceous sands as they relate to water-bearing properties as well as a discussion of the principal Cretaceous aquifer in the Charleston area. The drill holes used to construct the geologic sections consist of one continuously cored stratigraphic test hole, USGS-Clubhouse Crossroads #1 (fig. 1; DOR-37), and 13 municipal or private water wells. Geophysical logs were available for all of these drill holes, and cuttings were studied from the deepest water well, well CHN-635 located on Sullivans Island (fig. 2). Drillers logs were available for some of the water wells. Selected data from Stephenson's (1914) report on an additional well in Charleston city (CHN-14, fig. 2), and hydrologic data from an additional well in Mt. Pleasant (CHN-167, fig. 2), also are included. The geophysical logs used in this report are generalized digital versions of the original logs. Data curves on the original logs were read at 2-foot intervals to produce the digital files used to display the logs on the geologic sections. This procedure tends to omit some thin spikes, to clip some high-amplitude spikes, and to produce generally smoother curves than are present on the original logs. However, these digital logs remain adequate for studying the 50to 400-foot-thick units described in this report. Several combinations of gamma-ray and (or) electric logs (spontaneous potential with short-normal, single-point, or lateral resistivity) are used on the sections. The electric logs for drill holes CHN-172 and CHN173 have reversed spontaneous potential curves on which positive deflections (relative to the curve position opposite clay intervals) occur opposite freshwater sands rather than the conventional negative deflections. The datum for the geologic sections is the Cretaceous-Tertiary boundary; drill-hole depths are in feet. Conversion factors follow References Cited. Figures 1,2, and 3 are on the sheet containing sections A A' and B-B', whereas figures 4 and 5 are in this pamphlet. Acknowledgments