Abstract
In County Clare, western Ireland, the Namurian consists of a sandstone-shale sequence made up of repeated coarsening-upward, deltaic cyclothems, each 200 to 400 ft (60 to 120 m) thick. Sedimentologic details of these cyclothems indicate that they were deposited in deltas of high load input but low wave and littoral-drift influence; the ancient sequences are comparable to those of the modern Mississippi delta.
Syndepositional disturbances are an essential part of the cyclothems: the controlling and dominant feature is that of growth faults. These are scoop-shaped normal faults, that dip down current, and have a steep upper surface which passes downward into a flat shear plane. The insides of the scoops are filled almost entirely by overthickened sequences of distributary-mouth sandstones.
It is suggested that the growth faults of County Clare simply indicate the release of gravity forces inherent in a sediment column with an inverted density profile (sands over lighter, more mobile clays) like that in front of a prograding distributary. Release of the gravity forces by faulting allowed the entire delta mass to move downward and basinward, as an enormous “delta slip” abetted by continuous, rapid sedimentation.
The presence of growth faults in the sequence is both typical and symptomatic of the type of progradation.
