The Lorca basin, located within the tectonically evolving Betic Cordillera of southeastern Spain, developed as a rapidly subsiding basin in an open-marine setting during much of the late Miocene. The basin underwent sporadic phases of circulatory restriction with marked production and preservation of organic matter, culminating in evaporative sedimentation (latest Miocene). An organic geochemical study using biomarker analyses clarifies the degree of mixing between inflowing water sources (both marine and nonmarine), gives clues to the complex and variable circulation within the water and helps determine the extent of basinal hypersalinity. As the water in this basin evolved toward evaporative conditions, a number of organic-rich depositional phases (>25 wt.% TOC), here termed ''pre-evaporative,'' took place. During the early parts of these phases, the upper water is interpreted as nutrient rich and comparatively normal, and the bottom water as anoxic. This was followed by rising salinities and short phases of evaporite formation. Such a sedimentary history may serve as a model for understanding of evaporite-related source rocks and related hydrocarbons in other areas of the world.
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