Incipiently drowned platform deposit in cyclic Ordovician shelf sequence: Lower Ordovician Chepultepec Formation, Virginia

Conference · Tue Mar 01 04:00:00 EST 1983 · Am. Assoc. Pet. Geol., Bull.; (United States)

OSTI ID:6374815

Bova, J A; Read, J F

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/1000 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. 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. Locally, there are upward shallowing sequences, of basal wackestone/mudstone to calcisiltite to bioherm complexes (locally with erosional scalloped tops). Following each submergence, carbonate sedimentation was able to build to sea level prior to renewed submergence. Large submergence 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.

• Research Organization: Mobil Exploration and Producing Services Inc., Dallas, TX
• OSTI ID: 6374815
• Report Number(s): CONF-8304200-
• Conference Information: Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Journal Volume: 67:3
• Country of Publication: United States
• Language: English

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02 PETROLEUM
020200* -- Petroleum-- Reserves
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58 GEOSCIENCES
580100 -- Geology & Hydrology-- (-1989)
CARBONATE ROCKS
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FEDERAL REGION III
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOLOGIC HISTORY
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NORTH AMERICA
ORDOVICIAN PERIOD
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USA
VIRGINIA