The Gulf Coast of the United States is a passive-margin basin that contains a very thick accumulation of sediment. Drilling reveals that the base of the sedimentary succession in this basin consists of redbeds. These are overlain by a thick layer of evaporite which, in turn, is overlain by deposits composed predominantly of sandstone and shale. In some strata, the sandstone occurs in channels and contains ripple marks, and the shale contains mud cracks. In other strata, the sandstone and shale contain fossils of marine organisms. The sequence contains hardly any conglomerate or arkose. Be a sedimentary detective and explain the succession of sediment in the basin. (D)

The first step is to identify the depositional environments associated with each sedimentary layer based on their characteristics. 1. Redbeds: These are continental deposits, which are typically formed in terrestrial environments such as floodplains, fluvial plains, or desert settings. 2. Evaporite: These are formed in environments with high evaporation rates and limited water inflow, such as playas, lagoons, or shallow marine settings. 3. Sandstone and shale: These can be formed in a variety of environments, but the presence of ripple marks, mud cracks, and marine fossils in different strata suggests a coastal or shallow marine environment with transitions between sub-aerial (exposed to air) and sub-aqueous (submerged) conditions.

Based on the observed sedimentary layers and depositional environments, the succession of sediment in the basin can be explained as follows: 1. Initial Stage (Redbeds): Terrestrial deposition in a floodplain, fluvial plain, or desert setting, which accumulated over time at the base of the basin. 2. Second Stage (Evaporite): As the basin continued to subside, it became periodically or partially submerged, forming a shallow marine or lacustrine setting with high evaporation rates, leading to the deposition of evaporite minerals. 3. Third Stage (Sandstone and shale): The basin continued to subside and was periodically exposed to open marine conditions, leading to the deposition of sandstone and shale layers. The different strata within this layer suggest that the environment alternated between sub-aerial (creating ripple marks and mud cracks) and sub-aqueous (preserving marine fossils) conditions. The absence of conglomerate and arkose indicates a limited input of coarse sediments and a mature source area.

Based on the analysis of the sediment layers and depositional environments, the sedimentary succession in the Gulf Coast passive-margin basin indicates a progressive subsidence and shift from terrestrial to marine environments. The basin initially formed with continental redbeds deposits, followed by evaporite deposits in a lacustrine or shallow marine setting, and finally transitioned into a coastal or shallow marine environment characterized by alternating exposures leading to the deposition of sandstone and shale layers with ripple marks, mud cracks, and marine fossils.