Heterozoan carbonate lithofacies and sequence stratigraphy: a study of Pliocene strata of the Agua Amarga basin, southeastern Spain

Abstract

This study of Pliocene heterozoan carbonates in the Agua Amarga basin southeastern Spain seeks to increase understanding of factors controlling facies distribution and sequence stratigraphic stacking in heterozoan carbonates. Geologic mapping and stratigraphic sections reveal a dominantly 0.6º to 4.0º basinward-dipping substrate at high elevation, which is terraced (marine erosion) and incised (subaerial erosion). In heterozoan deposits, paleotopography influenced lithofacies distribution. Focusing of offshore-derived currents favored formation of in situ rhodolith banks near and between slope breaks in general and bryozoan packstones to grainstones specifically near concave slope breaks. Shallowly sloping substrate at high elevation resulted in deposition mainly during relative sea-level rise. In the Carboneras basin, steeply sloping substrate at high elevation and accommodation at low elevation resulted in deposition mainly during relative sea-level fall and lowstand. Data combined from the Agua Amarga and Carboneras basins show seven high-amplitude, high-frequency relative sea-level cycles interpreted as glacioeustatic cycles. Offset of this study's curve from the global glacioeustatic curve indicates the study area underwent subsidence then uplift of 62 to 153 m during or since Pliocene deposition. In the Agua Amarga basin, there are two sequences of heterozoan deposits (rhodalgal and molechfor lithofacies; c.f. Carannante et al., 1988) and two sequences of mixed heterozoan-siliciclastic deposits. For heterozoan Sequence 1, water-depth-sensitive facies formed retrogradational stacking patterns, and occurred late during relative sea-level rise near the highstand turnaround. In contrast, traditional photozoan carbonate models predict aggradation or progradation near highstand. This disparity can be explained by commonly low sediment-accumulation rates for heterozoan carbonates. For heterozoan Sequence 2, water-depth-sensitive facies (e.g. in situ oyster banks) formed retrogradational stacking patterns as sea level rose. Time equivalent deposition was precluded at these elevations in the Carboneras basin by steeply sloping substrate. The two mixed heterozoan-siliciclastic sequences are composed of metamorphic clast conglomerates and sandstones marking the beginning of input of metamorphic rock fragments into the study area, likely resulting from filling of the adjacent Nijar basin. The distribution of lithofacies, which fine downdip, is dictated by water depth relative to tidal range and wave base. Transgressive beach gravels of Sequence 3 are retrogradationally stacked.