Evolution of Early to Middle Cambrian Carbonate Platform, Southwest Virginia Appalachians

R. J. Barnaby, J. F. Read

Three core transects through the Early to Middle Cambrian continental shelf margin (Shady-Rome sequence) allow its evolution to be established. The sequence was initiated on a drowned clastic shelf to form a ramp with downslope mud mounds. Overall subsidence rates are estimated to be 5-10 cm/1,000 years.

Mud mounds consist of rare Archaeocyathid reefs that cap stromatactis MS/WS core facies and form upper parts of shallowing-upward sequences whose basal portion is shale and nodular bedded limestone. Individual mounds (1-30 m thick) are laterally extensive; some are over 3.5 km wide. Depositional slopes on the flanks range from < 0.5° to > 5.0°.

Three periods of backstepping of the mud mound sequences resulted from sea level fluctuations (average period ^approx 2 m.y.). Following the second backstep, regressive shelf sands and basinal turbidites formed. Later transgression led to development of mud mounds and patch reefs above the sands.

Sea level fall during Rome regression caused widespread deposition of cyclic red beds and carbonates on the slowly subsiding shelf interior. On the more rapidly subsiding outer platform where subsidence rates exceeded sea level fall, an algal reef-rimmed shelf formed landward of earlier ramp buildups. Seaward of the shelf were thick deposits of peri-platform talus and foreslope sands.

Small-scale shallowing-upward sequences record 10⁴ to 10⁵-year sea level fluctuations superimposed on longer term cycles. The platform was exposed during major regressions, resulting in meteoric diagenesis of the platform as well as the off-platform talus, which provided conduits for deeply circulating meteoric fluids.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.