Development of High-Energy, Tidally Modulated, Barred Shoreface Deposits: Kimmeridgian-Tithonian Sandstones, Weald Basin, Southern United Kingdom, and Northern France
The influence of tides on the sedimentology of wave-dominated shorefaces has been emphasized in recent studies of modern shorelines and conceptual models, but few ancient examples have been reported to date. The sedimentologic character and stratal architecture of Kimmeridgian-Tithonian shallow-marine sandstones from the Weald Basin (southern England and northern France) are described using a combination of outcrop and subsurface data, and interpreted as the deposits of high-energy, tidally modulated, wave-dominated, barred shorefaces. The sandstones occur as a series of upward-coarsening successions 3-20 m thick. The lower part of each succession consists of variably stacked, very fine- to fine-grained sandstone beds and mudstone interbeds that are moderately to intensely bioturbated by a mixed Skolithos and Cruziana ichnofacies. Event beds contain hummocky cross-stratification. This lower part of the succession is interpreted to record deposition on the sub-tidal lower shoreface, between effective storm wave base and fairweather wave base, and forms significantly thinner intervals (relative to the total thickness of the shoreface succession) than in typical wave-dominated shoreface successions. The upper part of each succession comprises cross-bedded, medium- to coarse-grained sandstones containing mud drapes, mudclast lags and wave-rippled surfaces (including interference ripples) that mantle the erosional bases of trough cross-sets. Bioturbation is patchy, and constitutes a low-diversity Skolithos ichnofacies. High-resolution seismic data indicate that the cross-bedded sandstones are arranged into small clinoform sets, interpreted as barforms, superimposed on steeply dipping (up to 5°) clinoforms extending through the upper part of the succession. These deposits are interpreted to record shallow sub-tidal and intertidal deposition on the upper shoreface. Each upward-coarsening succession therefore represents an upward-shallowing shoreface parasequence, which is typically truncated by an overlying bioclastic lag developed during transgression. Regional well correlations indicate that the shoreface parasequences form laterally extensive sheets fringing the northern margin of the basin. The pervasive evidence of tidal processes is attributed to amplification of tidal currents, due to paleobathymetric constriction of the tidal wave as it propagated from the Tethys Ocean into the adjacent shallow Weald Basin.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019