Resedimented Conglomerates in the Bottomsets of Gilbert-type Gravel Deltas

Abstract

ABSTRACT A marine, mass-flow-dominated, fan-delta sequence (Espiritu Santo Formation, Pliocene, Vera basin, southeastern Spain) and its resedimented conglomerate beds are described and interpreted. The studied sequence from the base upward contains 1) shelf mudstones (Cuevas Formation); 2) an up to 40-m-thick sequence of sand and conglomerate sheets with primary dip of up to 10° (lower delta slope, LDS); 3) up to 40 m high, planar to tangential delta foresets with primary dip of up to 25° (upper delta slope, UDS). Most of the LDS conglomerates have a matrix of lime mud (now recrystallized), silt, sand, granules, and pebbles containing scattered, weakly imbricated cobbles. There is little change in texture in the downcurrent direction, but the grain size decreases slightly. Conglomerate beds thicker than 2 m are mostly due to amalgamation, as inferred from remnants of mud-layer horizons occurring within these beds. Conglomerates are dominantly nongraded, although at the base, a few centimeters of thin, inversely graded zone may be present. Some beds show in their distal segments coarse-tail, normal grading covered by diffusely stratified conglomerate. Good exposures indicate that gravel deposition started just beyond the slope break (zone of hydraulic jump). The grain fabric suggests laminar-flow behavior of a plastic-viscous gravel mass (debris flow with dominantly frictional strength). The flat, eroded top of the gravel beds, and the presence of cobble-sized, imbricated clasts there suggest reworking by a strong current, probably at the time of their deposition. It is therefore suggested that file LDS conglomerates originate from the transformation of a delta-front slide either into a combination of high-density turbulent flow and debris flow (lower part), or into an initially fully turbulent flow. In the latter case, the debris flow would have originated from gravity transformation of the turbulent flow (Fisher 1983). In both ca es, the faster-moving turbidity current enhances the mobility of the debris flow. The LDS conglomerate beds are intercalated with dominantly structureless (pebbly) mudstone beds which are locally laminated (suspension sedimentation). The structureless mudstones probably originate from a combination of bioturbation, dewatering, resedimentation, and deformation of overriding, gravelly sediment flows. The UDS conglomerates contain clast- and matrix-supported beds which are dominantly nongraded. The matrix consists of some (recrystallized) lime mud, silt, sand, and small pebbles. These beds are inferred to have been deposited by debris flows similar to those of the LDS, but with relatively higher strength values, as inferred from the steep (25°) depositional slope. The tangential geometry of many UDS foreset beds has been explained by flow transformation (dilution) and acceleration (decrease of flow viscosity), enhancing its mobility. The regular occurrence of LDS conglomerate beds within bottomset fines is a modification of the classical Gilbert-type delta. Therefore, the term bottomset-modified Gilbert-type delta is applied to illustrate the presence of major delta-front failures.