Sequence model for coastal-plain depositional systems of the Upper Triassic (Betic Cordillera, southern Spain)

Abstract

The deposits of the Upper Triassic of the Subbetic Zone (Betic Cordillera) offer an example of semi-arid fluvial deposits passing into shallow-water facies through a mud-flat environment. From the study of the vertical changes in facies, a sequence model is proposed for the Upper Triassic succession, relating, for each systems tract, the changes in sea level, the accommodation space and the evolution of the facies within a coastal-plain depositional system. The lowstand depositional system is made up of a package of thick, amalgamated sandstone strata limited below by an erosive surface with little incision. These are sheet-flood deposits and channel fills which form part of an extensive alluvial system in which wide but shallow water courses appear, with the development of extensive sand bars. These sandbodies from the lowstand phase often contain mudrock intraclasts, plant remains and erosive surfaces. The accommodation space being extremely limited, the preservation of the floodplain or mud-flat deposits and palaeosols is practically nil. The result is an amalgamation of extensive sandbodies with fine intercalated claystone levels. The transgressive depositional systems consist mainly of a red claystone series, although the first sediments of these systems are composed of sandstone with major claystone intercalations. These lower sandy levels of the transgressive phase do not exceed 50 cm in thickness and usually contain burrows and small-scale sedimentary structures. In the intermediate part of the red claystone series of this transgressive episode of saline mud-flat facies, there are sandstone levels of a little-developed fluvial system with channels which migrate laterally due to the rise of the base level. In addition, the predominance and the great development of the red claystone facies reflect the increase in the accommodation space, which permitted a greater accumulation of sediments during a transgressive phase than during the lowstand phase. In the upper part of the transgressive depositional systems, beds of calcrete and carniolar limestone are common, indicating a decreased sedimentation rate. The generation of accommodation space begins to slow down, leading to the enlargement of sandbodies corresponding to terminal-fan deposits (Friend, 1978). These developed over a flat topography where the flow scattered and expanded laterally. In the highstand phase, the accommodation space reduced even more and the sedimentation rate is lower, favouring a great development of pedogenic carbonates. Furthermore, sand deposition was reduced or nil and the gradient of the slope was low, so that marine floods over the coastal plain were more frequent, permitting the precipitation of carbonates and sulphates in coastal salt pans.