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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 56 (1972)

Issue: 12. (December)

First Page: 2392

Last Page: 2414

Title: Depositional History of Carboniferous (Middle Visean) Limestones from Bristol and Parts of South Wales

Author(s): W. C. MacQuown, Jr. (2), T. W. Bloxam (3)


The mid-Visean Clifton Down Limestone and parts of adjacent formations on the Gower Peninsula of southern Wales can be subdivided into informal lithologic units with dominant microfacies that reflect both minor cyclic deposition and the effect of terrigenous influx toward the type locality at Bristol, England. These units also contain distinctive biotic and geochemical constituents that support paleoenvironmental interpretations.

Carbonate sediments were deposited on a shallow subsiding shelf under poorly oxygenated conditions, in a warm humid environment subject to variable physical energy, as indicated by the presence of abundant algae, relatively dark color, a relatively thick carbonate sequence without evaporites, and microfacies ranging from low-energy fine pelmicrites to high-energy coarse oointrasparudites.

The seemingly complex sequence of lithologic units can be explained by postulating concurrent local regression, including seaward oolitic sand barrier migration and regional transgression, including slow basin subsidence. The oolitic barrier is thought to have been formed by waves and related longshore currents on a slope break above a structural hinge line between a shallow shelf and a slightly deeper geosynclinal basin in the present Bristol Channel area. Restriction of the shallow shelf between the barrier and St. George's Land, about 30 mi north, created a back-barrier, tidal-flat environment. Subtidal, intertidal, and questionable minor supratidal subenvironments are indicated by tidal-channel ooliths, angular intraclasts, and algal-mat fragments possibly derived from mudbanks, s me of which may have been subaerially exposed.

Distal prodeltaic facies, with varying terrestrial influx through time, are thought to have resulted from the lateral swinging or intermittent progradation in a known delta complex north and east of Bristol, beyond the area of the present study.

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