Abstract: Progressive Changes in Facies Succession Motifs in Continental Strata--The Facies Model Concept Revisited

RAMON, JUAN CARLOS and TIMOTHY A. CROSS,

Oligocene-Miocene strata in the Middle Magdalena Basin, Colombia, contain deposits of fluvial channels, crevasse splay complexes, wet and dry floodplains and lakes. Studies of outcrops, cores, and well logs reveal regular, progressive changes in component facies plus their proportions from single and multiple facies tracts. Removed from their stratigraphic context, the observed facies successions of multiple facies tracts could be grouped into a few static facies succession motifs of facies models. Similarly, the variations in facies components and proportions of single facies tracts could be grouped into a few facies models of different river or floodplain types.

Channel sandstones change progressively from blocky, amalgamated trough-cross stratified sandstones without fining-up caps to fully preserved point-bar successions. Trends in trough-cross stratification set thickness, grain size, porosity and oil saturation are parallel to facies changes. Crevasse splays change from disorganized heterolithic alternations of mudstones and sandstones with bioturbation and dewatering structures dominant at the top to well-organized coarsening-up and thickening-up complexes with occasional crevasse channels with dewatering and bioturbation concentrated at the base. Amalgamated channel sandstones are overlain by thin (less than or equal to 0.7 m) floodplain mudstones. Overlying lake deposits are uncommon and are replaced by a thin degradational soil horizon. Fully preserved point-bar complexes are associated with thick floodplain mudstones which generally preserve overbank clinoforms of crevasse splays. The overlying lake mudstones are succeeded by thick coarsening-up crevasse complexes.

The geomorphic components of similar environments which enter the stratigraphic record change through time. We can explain these changes in terms of increasing and decreasing accommodation:sediment-supply ratios during stratigraphic cycles, From the perspective of facies models, these variations in facies components within facies succession motifs are disorganized sedimentologic noise. By contrast, these variations in facies components are regular and organized and contain considerable information from a stratigraphic perspective.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria