THROMBOLITE FABRICS IN THE CONOCOCHEAGUE FORMATION OF WESTERN MARYLAND: DIAGENETIC ALTERATION OF ORIGINAL STROMATOLITIC FABRICS

The Conococheague Limestone of Western Maryland is typical of Lower Paleozoic carbonates in that it contains well-developed thrombolites: mounds meters to tens of meters in diameter with internal clotted fabrics. These structures are interpreted to be microbial deposits but are rather enigmatic. Thrombolites within the Conococheague Limestone grade laterally and vertically into stromatolites. The clotted thrombolitic fabric is commonly found deforming well-defined stromatolitic lamination and in between well-defined layers as well. In these instances, clotted thrombolite fabrics are always associated with tubular, spar-filled disruptions, and differences in color due to textural differences. It is possible to trace stromatolite layers through the transitions in some places.

These observations suggest that, in some cases, thrombolites may simply be stromatolites that have been burrowed (or bored) and subject to aggrading neomorphism producing the color and textural differences. Recent research on modern Bahamian “stromatolites” (Plavansky and Ginsburg, 2009) has shown that whereas stromatolitic layering dominates the topmost areas of certain Bahamian stromatolites, physical and biological processes such as recrystallization and bioturbation distort the layers in lower sections of stromatolites producing a notable thrombolitic fabric. In the Conococheague Limestone these process apparently also effected stromatolites as well. Identifying an altered stromatolite disguised as a thrombolite requires clear remnants of layered fabrics, lateral or vertical gradation from layered to clotted fabrics and structures reminiscent of a stromatolite. In the absence of these criteria, it may be difficult to accurately assess the genesis of a thrombolite.