SEQUENCE MODEL FOR COASTAL-PLAIN DEPOSITIONAL SYSTEMS OF THE UPPER TRIASSIC (BETIC CORDILLERA, SOUTHERN SPAIN)

The deposits of the Upper Triassic of the Subbetic Zone (Betic Cordill era) offer an example of semi-arid fluvial deposits passing into shall ow-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 wi thin a coastal-plain depositional system. The lowstand depositional sy stem is made up of a package of thick, amalgamated sandstone strata li mited below by an erosive surface with little incision. These are shee t-flood deposits and channel fills which form part of an extensive all uvial 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 su rfaces. The accommodation space being extremely limited, the preservat ion of the floodplain or mud-flat deposits and palaeosols is practical ly nil. The result is an amalgamation of extensive sandbodies with fin e intercalated claystone levels. The transgressive depositional system s consist mainly of a red claystone series, although the first sedimen ts of these systems are composed of sandstone with major claystone int ercalations. These lower sandy levels of the transgressive phase do no t exceed 50 cm in thickness and usually contain burrows and small-scal e sedimentary structures. In the intermediate part of the red clayston e series of this transgressive episode of saline mud-flat facies, ther e are sandstone levels of a little-developed fluvial system with chann els which migrate laterally due to the rise of the base level. In addi tion, the predominance and the great development of the red claystone facies reflect the increase in the accommodation space, which permitte d a greater accumulation of sediments during a transgressive phase tha n during the lowstand phase. In the upper part of the transgressive de positional systems, beds of calcrete and carniolar limestone are commo n, indicating a decreased sedimentation rate. The generation of accomm odation space begins to slow down, leading to the enlargement of sandb odies corresponding to terminal-fan deposits (Friend, 1978). These dev eloped over a flat topography where the flow scattered and expanded la terally. In the highstand phase, the accommodation space reduced even more and the sedimentation rate is lower, favouring a great developmen t 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.