Controls on facies distributions within rift-lakes: Examples from the modern (Lake Tanganyika, Africa) and the ancient (Bisbee Basin, Arizona).

Abstract

Lake Tanganyika is the most widely cited modern analog for interpreting ancient rift-lake deposits. The lake consists of a series of half-grabens linked by a series of structural horsts. This asymmetric geometry allows classification of the basin margin into four distinct types of lake margins: escarpment (footwall), accommodation zone (structural horsts), axial, and hinged (hangingwall). Sedimentologic study of six study sites that represent all four margin types indicates that margin geomorphology, such as (e.g. depositional slope, and drainage size) and littoral sedimentology (e.g. occurrence of shore-parallel facies belts, and presence of certain environments such as winnowed surfaces, fan-deltas) differ significantly among the structural margin types. Compositional differences among the sites also reflect the structurally controlled variability in sedimentary processes. Total Organic Carbon (TOC) and Rock-Eval pyrolysis data show that relatively rich, hydrogen-rich organic matter typically accumulates along the accommodation-zone and escarpment margins, whereas typically lean, oxidized organic accumulates along axial and hinged margins. Sand composition among the margin types of Lake Tanganyika clusters tightly using the Gazzi-Dickinson point-counting methodology, confirming that provenance variability is generally minor. However, significant compositional differences among the tectonic margins appear when these data are cast using the traditional point-counting methodology. These compositional differences are primarily controlled by transport and depositional processes that vary among the sites. The Apache Canyon Formation, a mixed carbonate-clastic lacustrine sequence deposited within the extensional Bisbee basin of southeastern Arizona, provides a test of predictive capacities of the basin-fill model developed from the Lake Tanganyika data. Three measured sections, located along a transect perpendicular to the basin-bounding fault, are generally consistent with facies predictions developed from the Lake Tanganyika model. The observed first-order facies contrasts among the measured sections support the inference that the Apache Canyon Formation was deposited within an ancient half-graben lake basin. Although the actualistic model predicted adequately facies assemblages, however, the facies stacking patterns were less successfully predicted by the model. The discrepancies likely relate to temporal changes in tectonic and climatic effects that are not captured within a model based on modern data.