Middle Ordovician Carbonate Ramp Deposits of Central Appalachians

Robert V. Demicco

Middle Ordovician carbonates exposed in Maryland and Pennsylvania can be divided into six facies, each a few tens to hundreds of meters thick: (1) cyclic, meter-scale, alternating thin-bedded to massive limestones and mud-cracked, stromatolitic laminites; (2) thick-bedded to massive skeletal wackestones containing diverse fauna; (3) cross-stratified skeletal-oncoid grainstones; (4) graded, thin-bedded limestones with diverse fauna and internal planar lamination or hummocky cross-stratification; (5) nodular, thin-bedded limestones; and (6) shaly, thin-bedded to laminated limestones containing rare breccia beds. These facies are interpreted as deposits of: (1) tidal flats; (2) open, bioturbated muddy shelf; (3) lime-sand shoals; (4) below normal wave-base shelf; (5) deep ra p; and (6) basin.

Palinspastic reconstructions of facies distribution in Maryland and Pennsylvania suggest that these facies developed during flooding of a carbonate ramp that deepened northeastward into a foreland basin. This northern depocenter of the Middle Ordovician Appalachian foreland basin is notably different than its southern counterpart in Virginia and Tennessee. Large skeletal bioherms did not develop on the northern carbonate ramp, where only one onlap package exists. Thus, although the record of the foundering of the passive Cambrian-Ordovician carbonate shelf is grossly similar in the southern and central Appalachians, there are several significant differences. The overlying Martinsburg Formation contains deep-water facies and taconic-style thrust sheets in the central Appalachians, which suggests that the two depocenters may have had different tectonic settings.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.