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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 52 (1968)

Issue: 3. (March)

First Page: 554

Last Page: 555

Title: Facies Changes Developed During Filling of a Deep Basin: ABSTRACT

Author(s): Roger G. Walker

Article Type: Meeting abstract

Abstract:

A sequence of lithofacies developed during basin filling has been studied in a 150-m regression cyclothem in the Pennsylvanian of north Devon, England. The cyclothem can be divided from bottom upward into eight units.

Unit 1 (35 m) contains only black mudstone.

Unit 2 (12 m) contains 140 sharp-based, graded siltstone-turbidites, each about 2 cm thick. Sole marks indicate a wide spread of current directions, with a mean toward the south.

Unit 3 (9 m) contains regularly interbedded structureless muddy siltstone, and cross-laminated siltstone beds which usually have gradational bases. Six beds are sharp-based and bear sole marks.

Unit 4 (12 m) contains interbedded siltstone-turbidite and cross-laminated siltstone similar to those in units 2 and 3. There are also two channels at least 2 m deep and filled with thicker turbidite.

Unit 5 (18 m) is composed dominantly of muddy siltstone with irregular cross-lamination. Sharp-based siltstone-turbidite is rare, and dies out upward. Gradational-based

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cross-laminated sandstone increases in abundance upward.

Unit 6 (47 m) contains gradational-based cross-laminated sandstone, irregularly interbedded with gray mudstone and cross-laminated siltstone. The sandstone beds become thicker and more abundant upward.

Unit 7 is a channel cut into the top of unit 6. The minimum depth is 15 m, and minimum width 100 m. The lower 5 m of fill consists of graded siltstone-turbidite, and the upper part is identical with unit 6.

Unit 8 (15 m) consists of trough-cross-bedded and cross-laminated coarse sandstone, with few thin mudstone beds. The sandstone is of nearshore, possibly estuarine, origin.

The whole sequence indicates gradual basin filling. The turbidite-filled channel near the top is cut into "shelf-type" sediments, and probably acted as a passage for turbidity currents flowing farther into the basin. There is no evidence of slumping in the cyclothem, and the turbidity currents probably originated directly from rivers carrying at flood stage a high proportion of silt and mud in suspension into the basin.

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