CHARACTERIZATION OF ORGANIC-MATTER FROM A RESTRICTED EVAPORATIVE SEDIMENTARY ENVIRONMENT - LATE MIOCENE OF LORCA BASIN, SOUTHEASTERN SPAIN/

The Lorca basin, located within the tectonically evolving Betic Cordil lera of southeastern Spain, developed as a rapidly subsiding basin in an open-marine setting during much of the late Miocene. The basin unde rwent sporadic phases of circulatory restriction with marked productio n and preservation of organic matter, culminating in evaporative sedim entation (latest Miocene). An organic geochemical study using biomarke r analyses clarifies the degree of mixing between inflowing water sour ces (both marine and nonmarine), gives clues to the complex and variab le circulation within the water and helps determine the extent of basi nal hypersalinity. As the water in this basin evolved toward evaporati ve conditions, a number of organic-rich depositional phases (>25 wt.% TOC), here termed ''pre-evaporative,'' took place. During the early pa rts of these phases, the upper water is interpreted as nutrient rich a nd comparatively normal, and the bottom water as anoxic. This was foll owed by rising salinities and short phases of evaporite formation. Suc h a sedimentary history may serve as a model for understanding of evap orite-related source rocks and related hydrocarbons in other areas of the world.