Internal architecture of mixed tide- and storm-influenced deposits: an example from the Alcântara Formation, northern Brazil

Abstract

The uppermost portion of the Itapecuru Group is exposed in the eastern margin of the Sao Luis Basin, northern Brazil, where it consists of two units: the Alcantara Formation (Cenomanian) and the Cujupe Formation (Late Cretaceous-early Tertiary? [Rossetti, D.F., Truckenbrodt, W., 1997. Revisão estratigrdfica para os depósitos do Albiano-Terciário Inferior (?) na Bacia de Sao Luis (MA), norte do Brasil. Bol. Mus. Paraense Emílio Goeldi (Sér. Ciênc. Terra), in press]). The Alcântara Formation, which contains the large-scale structures discussed in this paper, consists of deposits attributed to mid- to upper-shoreface, foreshore, tidal channel, and lagoon/washover environments attributed to a regressive, barred shoreline. Several types of large-scale cross bedding (i.e., simple foreset, compound, mixed, undulatory, and intricately bounded) were recognized in the shoreface facies association. These structures are interpreted to record the interaction of storm and tidal processes. The storm influence is suggested by a combination of factors, mostly including: (a) the genetic association with other storm-generated sedimentary structures (i.e., swaley cross stratification and undulating parallel lamination with internal truncations); (b) the deposition on prominent surfaces formed by storm erosion, which are defined by large-scale, either symmetrical or asymmetrical scours arranged in a regular, repeating pattern; (c) the sedimentary features formed under combined (unidirectional and oscillatory) flow processes (e.g., compound/mixed bedding with superimposed either swaley cross sets or complexly truncating cross sets with highly undulating boundaries; large-scale, undulatory and intricately bounded cross beddings); and (d) the lateral change in structural styles within short distances, which records frequent modification from asymmetrical to symmetrical/nearly symmetrical bedform profiles (more likely to occur under storm-generated combined flows). The tidal signature is locally recognized by regularly spaced, thick/thin sandstone bundles defined by reactivation surfaces and/or mud drapes, which are attributed to tidal (ebb/flood) cycles. The analysis of paleocurrent distribution suggests that vigorous, southwestward-oriented storm flows interacted with local tidal currents on the shoreface to promote the landward transport of significant volumes of sand, which resulted in the large-scale cross stratification described in this paper. In addition, a secondary, southeastward-directed (oblique- to shore-parallel) combined flow would have periodically interacted with the main flow. The origin of this oblique- to shore-parallel flow is attributed to either the refraction of the main storm waves as they approached the paleocoastline or the interference of a separate storm episode, which competed with the major, landward-moving one. This complex storm flow pattern can be related to past penecontemporaneous seismic activity (i.e., tsunamis), as suggested by the paleogeography of the study area during the Cenomanian, combined with the structural history of the Sao Luis Basin and the sedimentary features recognized in the Alcantara Formation.