Depositional Settings of the basal López de Bertodano Formation, Maastrichtian, Antarctica

Abstract

In Snow Hill and Seymour islands the lower Maastrichtian, basal part of the L ópez de Bertodano Formation, rests on a high relief, erosive surface elaborated in the underlying Snow Hill Island Formation. Mudstone-dominated beds with inclined heterolithic stratification dominate the basal strata of the López de Bertodano Formation. They consist of rhythmical alternations of friable sandy- and clayeymudstone couplets, with ripple cross lamination, mud drapes, and flaser bedding. They are characterized by a marked lenticular geometry, reflecting the filling of tide-influenced channels of various scales and paleogeographic positions within a tide-dominated embayment or estuary. Major, sand-rich channel fills, up to 50-m thick, bounded by erosive surfaces probably represent inlets, located on a more central position in the estuary. Minor channel fills, 1- to 3-m thick, associated with offlapping packages with inclined heterolithic stratification probably represent the lateral accretion of point bars adjacent to migrating tidal channels in the upper estuary. Both types of channel fills bear relatively abundant marine fauna, are intensively bioturbated, and are interpreted as a network of subtidal channels. In southwestern Snow Hill Island, the minor offlapping packages have scarce marine fossils and bear aligned depressions interpreted as poor preserved dinosaur footprints. They represent the lateral accretion of point bars adjacent to intertidal creeks, probably located on the fringes of a mud-dominated estuary or embayment. The basal unconformity was produced by subaerial erosion; hence the inferred estuarine settings are consistent with the beginning of a new transgressive sedimentary cycle.