Laguna Mar Chiquita (central Argentina; ~latitude 31°S, longitude 63°W) provides an outstanding opportunity to examine organic facies development and petroleum source-rock potential in a modern thick-skinned foreland basin lake. In this case study, we define profundal, paleodelta, and lake-margin depositional environments based on trends in bathymetry and lake-floor sediment particle size. Sedimentary geochemical analyses indicate that organic carbon–rich muds accumulate in profundal environments during the extant lake-level highstand. The lateral variability of organic facies is minimal. The quality of organic facies is controlled by lake level and depositional environment, both of which dictate patterns of algal productivity, siliciclastic dilution, and early diagenesis. We present conceptual models of lacustrine source rocks in both thick-skinned and thin-skinned foreland basins based on modern analog data from both Laguna Mar Chiquita and other lakes in the central Andean foreland. Over relatively short time intervals (10 2 –10 4 yr), climatically driven water-level fluctuations influence the source-rock potential of these basins. Over time intervals >10 5 yr, contraction and lateral migration of the basin flexural profile control stratal stacking patterns and the potential for hydrocarbon play development.

Lake deposystems are commonly associated with retroarc mountain belts in the geological record. These deposystems are poorly characterized in modern retroarcs, placing limits on our ability to interpret environmental signals from ancient deposits. To address this problem, we have synthesized our existing knowledge about the distribution, morphometrics, and sedimentary geochemical characteristics of tectonically formed lakes in the central Andean retroarc. Large, active mountain belts such as the Andes frequently create an excess of sediment, to the point that modeling and observational data both suggest their adjacent retroarc basins will be rapidly overfilled by sediments. Lake formation, requiring topographic closure, demands special conditions such as topographic isolation and arid climatic conditions to reduce sediment generation, and bedrock lithologies that yield little siliciclastic sediment. Lacustrine deposition in the modern Andean retroarc has different characteristics in the six major morphotectonic zones discussed. (1) High-elevation hinterland basins of the arid Puna-Altiplano Plateau frequently contain underfilled and balanced-filled lakes that are potentially long-lived and display relatively rapid sedimentation rates. (2) Lakes are rare in piggyback basins, although a transition zone exists where basins that originally formed as piggybacks are transferred to the hinterland through forward propagation of the thrust belt. Here, lakes are moderately abundant and long-lived and display somewhat lower sedimentation rates than in the hinterland. (3) Wedge-top and (4) foredeep deposystems of the Andean retroarc are generally overfilled, and lakes are small and ephemeral. (5) Semihumid Andean back-bulge basins contain abundant small lakes, which are moderately long-lived because of underfilling by sediment and low sedimentation rates. (6) Broken foreland lakes are common, typically underfilled, large, and long-lived playa or shallow systems.

Abstract Modern lake basins set within active continental rifts provide useful analogs for exploration efforts in ancient extensional basins that are known to be rich in hydrocarbons. Lake Albert is one of the Great Lakes of Africa and is located at the northern end of the western branch of the East African rift system. This large, but comparatively shallow, eutrophic, and probably geologically ephemeral lake basin serves as an end-member example of the modern tropical lake systems that occupy this extensional province. Seismic reflection and gravity data sets indicate that the basin contains a maximum of 5 km (3.1 mi) of synrift, dominantly lacustrine sedimentary fill, in two subbasins separated by a midbasin high. In contrast to other large rift basins in the western branch of the rift valley, the Lake Albert Rift is not a highly asymmetrical half-graben basin, but instead has subsided nearly symmetrically and continuously in the late Cenozoic along two extensive boundary fault systems on either side of the basin. Seismic sequences from across the basin were correlated to borehole stratigraphy from a deep well drilled on the Ugandan margin. These observations suggest that the basin has experienced a long-term change from a continuously open lacustrine, possibly deep lake system in the Miocene or early Pliocene, to an alternating shallow lacustrine and fluvial system in the mid and late Pleistocene. This history of basin evolution has led to the development of a rich hydrocarbon system.