The Transgression of a Hard Substrate Shelf: The Llandovery (Lower Silurian) of the Welsh Borderland

Abstract

ABSTRACT Llandovery rocks of the Welsh Borderland document sedimentation on an irregular hard rock shelf during an eastward transgression. Two transgressive phases are recognized. During the first, a southward facing peninsula was flanked to the east (shore-wards) by stream-fed beaches and trapped estuarine sediments; and to the west (seawards) by a tide-swept bank undergoing in situ grading. To the south, westward prograding deltas countered the initial transgression. During the second phase, nearshore tidal and wave activity shifted eastwards· Thickness considerations suggest that offshore, muds were deposited from seawater with a suspended mud content closer to the present waters of the North American East Coast outer shelf than the more muddy North Sea. Storms generated many of the laminated sandstones and coquinas interbedded with the muds. The shelf attained a climax graded condition in late Llandovery times. The main factors controlling sedimentation were topography, tides, waves, fluviatile influx and storms. Many aspects of Llandovery sedimentation are only observed individually in other transgressive sequences.