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The AAPG/Datapages Combined Publications Database

CSPG Special Publications

Abstract


Shelf Sands and Sandstones — Memoir 11, 1986
Pages 338-338
Symposium Abstracts: Storm-Dominated Shelves

Storm- and Wave-Dominated Sandstones on an Extensive Ordovician Shelf: Abstract

Patrick J. Brenchley1

Abstract

The Monte da Sombadeira Formation is a sequence of storm- and wave-affected sandstones (15 to 190 m thick) interbedded with, and lying between “shelf” mudstones. The formation can be recognized throughout central Iberia, and outcrops have been studied in an area of over 40 000 km2. Hummocky cross-stratification (HCS) is common throughout the whole outcrop area, but proximal and distal facies may be distinguished. The most proximal facies (?shoreface) has amalgamated HCS within a continuous sandstone sequence. Another proximal facies (?inner shelf) has laterally discontinuous beds with erosional bases, some of which are hummocky; tops of beds are erosional, HCS may have steep dips, amalgamation of beds is uncommon and there are associated wave ripples. The distal facies (100 km + distant) has beds with only slightly erosional bases, low angle HCS, non-erosional tops, mudstone partings between some beds, but common amalgamation, and rare wave ripples. The HCS, both proximally and distally can be scour and drape or accretionary, the latter generally being associated with smaller-scale structures. Some small-scale (< 5 cm) structures with apparent form discordant “wave” lamination have been found to be hummocky in form or related to polygonal (?interference) ripples. Important inferences from the above data are: 1. erosional bases to HCS beds may be of wave origin; 2. polygonal ripples and small-scale HCS suggest multidirectional oscillatory currents; 3. powerful currents carried sand more than 150 km from the nearest shoreline. Waves were capable of reworking the sand throughout the whole area, though the different intensity of waves determined the particular facies; and 4. amalgamation of sandstone beds can be proximal or distal and is a product, both of frequency of storm waves and the amount of mud available to mantle structures.


 

Acknowledgments and Associated Footnotes

1 Department of Geology, University of Liverpool, Liverpool, U.K.

Copyright © 2008 by the Canadian Society of Petroleum Geologists