--> ABSTRACT: Depositional Sequences, High-Frequency Sea Level Fluctuations and Active Tectonics in the Middle to Late Ordovician Appalachian Foreland Basin, Kentucky and Virginia, by Mike Pope, J. Fred Read; #91020 (1995).

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Depositional Sequences, High-Frequency Sea Level Fluctuations and Active Tectonics in the Middle to Late Ordovician Appalachian Foreland Basin, Kentucky and Virginia

Mike Pope, J. Fred Read

Detailed regional cross-sections of Middle-Late Ordovician strata in Kentucky and Virginia, based on drill cores, road cut exposures and wireline logs, provide a superb data base for documenting the various scales of sequences in a predominantly deep ramp succession deposited in a tectonically active foreland basin during dominantly moderate amplitude eustatic conditions. These cross-sections provide an ancient analog for ramps developed in similar settings, which may be important for exploration because of the interfingering of deeper basinal beds and potential grainy reservoir facies.

It contains two large scale sequences (each up to 300 m thick, ~10-15 m.y. duration) bounded by regional unconformities. Internally, these sequences are packaged into smaller-scale 3rd/4th order sequences (30-100 m thick): thin TST's of skeletal grainstones/packstones (sometimes absent) overlain by regionally extensive black shale or rhythmite; thick HST's of deep ramp facies with localized skeletal grainstone shoals over tectonic highs; and thin LST's of tidal flat facies or marine clastics, showing little evidence of emergence, located along active arches or basin margins. Within the smaller sequences are various types of meter-scale cycles (0-10 m thick) indicating moderate amplitude eustatic fluctuations under warm and temperate climatic conditions.

Barrier banks and downslope buildups adjacent to the underfilled foredeep are well developed in the early Middle Ordovician. Shoal water banks are restricted to active tectonic arches south of the migrating underfilled foredeep in the Later Middle Ordovician. The near absence of grainstone in the Late Ordovician possibly reflects a more gentle slope on the ramp and near overfilling of the southern portion of the basin with the deep basin far to the north. Locus of the potential grainy reservoir facies is determined by active tectonics while compartmentalization of the reservoir facies is a result of high-frequency eustatic fluctuations.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995