Tectonic and Climatic Controls on Lake Distribution and Environments in Afar from Miocene to Present
F. Gasse, 1990. "Tectonic and Climatic Controls on Lake Distribution and Environments in Afar from Miocene to Present", Lacustrine Basin Exploration: Case Studies and Modern Analogs, Barry J. Katz
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Located at the triple junction of the African, Arabian, and Somalian plates, the Afar depression has been subjected to intense and complex volcano-tectonic activity from the Miocene into the Holocene. The region also has experienced significant climatic changes. The chronology of ancient lacustrine sedimentation is based on K-Ar dating of associated volcanics, diatom biostratigraphy, and 14C ages.
Spreading of the Afar depression accounts for the migration of active sedimentary basins from the margins of the Afar triangle to the modern active segments (Lakes Asal and Afrera). Tectonic conditions have been favorable for the settlement of deep, closed lakes in central Afar since the late Pliocene (2.5 Ma). There, ancient lakes may not be related to present topography. Sediment distribution shows a southwest- to-northeast progression in age of opening of the modern basins and a change in direction of major active faults from about east-west to northwest at —1.4 Ma. Deformation of Holocene sediments and shorelines allows the direction and rate of recent vertical displacement to be estimated.
Rifting also has increased elevation differences and hydraulic gradients between the depression and surrounding highlands, thereby increasing the effectiveness of both surface waters and groundwater circulation. This explains the general increase in water depth since the Miocene.
The effects of climatic fluctuations have been superimposed onto the structural evolution of the depression. Because climate governs chemical weathering, it is partly responsible for the sedimentary facies, specifically whether detrital, biogenic silica, or chemical carbonate fractions dominate. At any given time, the ratio of precipitation to evaporation in the Afar and on neighboring plateaus is responsible for the presence or absence of lakes in the deepest basins. Major lacustrine stages are correlated both with evolution of other tropical lakes and with global climatic events.
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Lacustrine Basin Exploration: Case Studies and Modern Analogs
Lacustrine environments are a major contributor of petroleum source rocks. Lacustrine source rock prediction is, however, influenced by numerous, complex variables governing lake sedimentation. Current predictive capability can be improved by attempting to map essential climatic variables to limit in space and time the area of lacustrine source rock exploration. Climatic characteristics that govern lake occurrence and the potential for stratification have been investigated with a General Circulation Model of the atmosphere for the present and for the mid-Cretaceous. In this analysis, the distribution of areas with a positive water balance first is used as an indicator of the distribution of areas conducive to lake formation. Second, the distribution of areas that experience large annual climatic variations is used as an indicator of the distribution of lakes that are less likely to be stratified and, hence, less likely to be sites of high organic-carbon preservation. Four factors used to define large climatic variations include (1) seasonal temperature cycle in excess of 40°C; (2) seasonal temperature extreme of less than 4C°; (3) average seasonal differences in precipitation minus evaporation balance in excess of 5 mm/ day; and (4) distribution of mid-latitude winter storms. Evidence is presented to support the capability of climate models that add insight into lacustrine source rock prediction by simulating geographic regions conducive to lake development and to stratification and organic-carbon preservation