Architecture and sedimentary facies evolution in a delta stack controlled by fault growth (Betic Cordillera, southern Spain, late Tortonian)

Abstract

Tectonic control is revealed in various ways (synsedimentary deformation structures, facies, architecture) in the coarse-grained delta systems that developed in the southeastern margin of the Guadix Basin, an intramontane basin in the central sector of the Betic Cordillera, Spain, during the late Tortonian (Miocene). Vertical trends in the architecture of the deltaic succession (230 m thick) show changes in the stratal stacking pattern related to the variation in time in subsidence rates (in accommodation space). A period of high subsidence controlled by a normal growth fault began at the base of the succession, producing retrogradational units representing Gilbert-type delta systems onlapped by shallow platform calcarenites. A period of low subsidence followed, controlled by growth of a listric fault producing aggradational units representing shoal-water deltas capped by red algal biostromes. A non-subsidence period and decrease in accommodation space at the top of the succession (sediment supply remained constant) produced progradational deltas. The manuscript focuses on the part of the succession where the delta deposits show the effects of extensional tectonics, a listric growth fault and its rollover, on delta development. The progressive increase in accommodation space inherent in fault growth controlled the style of delta sedimentation in the river mouth. Gilbert-type deltas developed during periods of increase in subsidence rates and shoal-water deltas during periods of decrease in subsidence rates. Horizontal trends in the architecture of the delta lobes show changes in the stratal stacking pattern affected by differential subsidence and pre-existing basin-floor topography. Periods of increase in subsidence rates near the fault scarp correlate with accommodation space kept constant, thinning of the units and shoal-water deltas development over the rollover, away from the fault scarp.