Stephen J. Collins¹, John Howell¹, Adrian Hartley²

(1) University of Liverpool, Liverpool, United Kingdom
(2) University of Aberdeen, Aberdeen, United Kingdom

ABSTRACT: Shallow marine processes in extensional basins

Present models for sedimentation within extensional basins greatly under-represent the degree of complexity that is present in shallow marine systems. Such systems are typically very aggradational, may contain frequent unconformities and, exhibit complex interactions between structural derived topography and depositional process. A number of cases from different stages of rift evolution have been studied. Examples include both tide and wave dominated depositional systems.

In northern Chile, Neogene sediments are exposed in a half-graben and comprise over 400m of storm deposited sandstones interfingured with diatomites and fan delta facies. The section contains numerous intra-formational unconformities and is interpreted to have been deposited during rift-initiation.

In the Neuquén Basin, Argentina, 100m of aggradational, storm deposited shorefaces are observed on fault-block highs. These lack unconformites and were deposited in the rift-climax. Subsurface data from the Jurassic of the North Atlantic also shows wave-dominated shorefaces on hanging-wall highs. They exhibit a forced regressive nature and are interpreted to have been deposited during late stage rifting.

Jurassic sediments from Scotland, display strong tidal influences. In this case the structural topography has amplified the tidal wave. Key surfaces within the basin are understood to represent periods of fault block rotation, and changes in relative sea-level.

Examination of the key architectural elements provides an improved understanding of shallow marine facies and facies architecture within these basins. This helps to constrain the temporal and spatial evolution the basins, and to better understand the interplay of sediment supply and accommodation creation.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado