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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 428

Last Page: 429

Title: An Incipiently Drowned Platform Deposit in Cyclic Ordovician Shelf Sequence: Lower Ordovician Chepultepec Formation, Virginia: ABSTRACT

Author(s): John A. Bova, J. F. Read

Article Type: Meeting abstract


The Chepultepec interval, 145 to 260 m (476 to 853 ft) thick, in Virginia contains the Lower Member up to 150 m (492 ft) thick, and the Upper Member, up to 85 m (279 ft) thick, of peritidal cyclic limestone and dolomite, and a Middle Member, up to 110 m (360 ft) thick, of subtidal limestone and bioherms, passing northwestward into cyclic facies. Calculated long term subsidence rates were 4 to 5 cm/1,000 yr (mature passive margin rates), shelf gradients were 6 cm/km, and average duration of cycles was 140,00 years.

Peritidal cyclic sequences are upward shallowing sequences of pellet-skeletal limestone, thrombolites, rippled calcisiltites and intraclast grainstone, and laminite caps. They formed by rapid transgression with apparent submergence increments averaging approximately 2 m (6.5 ft) in Lower Member and 3.5 m (11.4 ft), Upper Member. These submergence increments may have resulted from small (about 1 m; 3.2 ft) relative sea level rise, coupled with subsidence due to loading by water and accumulating sediment. Progradation of tidal flat facies did not occur until subsidence had ceased. Tidal flats were shifted westward from 70 to 380 km (43 to 236 mi) after each submergence event, but in peritidal sequences, had sufficient time to prograde back into the shelf.

Deposition during Middle Member time was dominated by skeletal limestone-mudstone, calcisiltite with storm generated fining-upward sequences, and burrow-mixed units that were formed near fair-weather wave base, along with thrombolite bioherms (subwave base to wave agitated shallow water settings). Locally, there are upward shallowing sequences, (subtidal cycles) of basal wackestone/mudstone to calcisiltite to bioherm complexes (locally with erosional scalloped tops). Apparent submergence increments during Middle Member deposition averaged 7.0 m (23 ft), ranging up to 23 m (75 ft) in southeastern belts. Following each submergence, carbonate sedimentation was able to build to sea level prior to renewed submergence. Large submergence

End_Page 428------------------------------

events caused tidal flats to be shifted far to the west, and they were unable to prograde out onto the open shelf because of insufficient time before subsidence was renewed, and because the open shelf setting inhibited tidal flat deposition. The Middle Member represents an incipiently drowned sequence that developed by repeated submergence events. Such incipiently drowned shelf sequences are common but poorly documented in the geologic record. They punctuate many aggraded cyclic shelf sequences and may ultimately provide important information for construction of relative sea level curves for Paleozoic.

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