Reports
Selected examples of carbonate sedimentation, Lower Paleozoic of Maryland
1976, Reinhardt, J. and Hardie, L.A.
Guidebook 5
Introduction
The objectives of this two-day field trip are: 1) to visit localities representative of the Cambrian, Ordovician, and Silurian carbonate rocks in the central Appalachians; 2) to demonstrate similarity in depositional environments and patterns of sediment accumulation through a considerable period of geologic time, even though each stratigraphic unit poses unique problems of interpretation; and 3) to document a major carbonate facies change in Upper Cambrian rocks from west (Great Valley) to east (Frederick Valley). The first day we will focus on shallow-water sedimentation as portrayed in three temporally separate stratigraphic units in Washington County, Maryland; the second day we will concentrate on the evolution of a depositional basin at the edge of the Cambrian and Ordovician carbonate platform, Frederick County, Maryland.
The Great Valley section was an active area for research during the 40's and 50's. Numerous students from Johns Hopkins University (Neumann, 1950; Long, 1953; Sando, 1957) and Penn State University (Pelto, 1942; Folk, 1952; Rones, 1955; Donaldson, 1959) helped to develop a petrographic and biostratigraphic framework for these Cambrian and Ordovician rocks. This work together with the stratigraphic work of Stose, Bassler, Willard, and Butts, and the recent synthesis by Palmer (1971), has resulted in a well established framework for this thick sequence of carbonate rocks. A somewhat schematic cross section (Fig. 1) adapted from Donaldson (1969) presents some of the stratigraphic nomenclature and the gross thicknesses of the Cambrian and Lower Ordovician sediments.
Recent developments in comparative sedimentology and new concerns with lithogenesis and weathering of carbonate rocks make it imperative that these rocks come under new scrutiny. Not only can we learn valuable lessons about sedimentation on carbonate platforms--epeiric seas, we can also investigate compaction, cementation, and diagenesis of various closely related lithologies. Preservation of both primary and secondary structures is superior in these rocks; our task is to unravel the early, intermediate or late lithogenesis. Some of these questions can be answered in the field and questions pertinent to further investigation can be formulated.
