INTERNAL STACKING PATTERNS OF ANCIENT TURBIDITE SYSTEMS FROM COLLISIONAL BASINS

Abstract

Abstract The preliminary conclusions derived from the detailed study of some 25 ancient turbidite systems of collisional basins indicate that, at different hierarchical orders, basinward and landward shifts of sandy depositional zones and their related sediment transfer zones control the observed stacking patterns. Within each system, these zonal shifts produce a hierarchy of units which, from the smallest to the largest, include elementary facies tracts (EFTs), facies, facies associations and stages. EFTs, composed of a limited number of beds (generally less than 10), slack to form facies; facies stack to form facies associations; facies associations stack to form stages; and stages stack to form systems. In the latest stages of growth of some systems, the zones of sand deposition shift landward in adjacent delta or fan-delta complexes; in these cases, slope and basinal turbidite sedimentation may become essentially restricted to fine-grained sediments (channel-levee complexes or overbank wedges) which are more or less directly sourced by fluvial systems. High-frequency, cyclic forestepping-backstepping episodes of basinal sand deposition are recorded within each system and its component stages by successions of beds (EFTs) and facies (facies associations) which commonly are expressed by m- to dam-thick units showing overall thinning-and fining-upward trends. These trends are thought to be related primarily to the frequency of gravity flows and to cyclic variations in their individual volume. The origin of these high-frequency cyclic variations is still poorly understood. High-frequency cyclic stacking patterns can be recognized only on a case-by-case basis, through careful facies analysis and very detailed stratal correlation patterns. Locally, the stacking patterns of individual sandstone beds within EFTs or within sandstone facies composed of EFTs can be considerably modified by local depositional or structural relief, as well as by differential compaction. Retrogressive slumps produced by failure of poorly consolidated sediment stored in oversupplied or tectonically unstable basin margins are likely to be one of the main causes for the high-frequency cyclicity observed in the early stages of development of many turbidite systems formed during periods of relative sea-level lowstand. The same type of cyclicity may be controlled by changes in shelfal accommodation and climate when slump scars are progressively reached by the deep-water extensions of prograding delta complexes during subsequent periods of relative sea-level rise. Growing evidence suggests that in high-relief physiographic settings, high-frequency climatic changes can trigger catastrophic, subaerially derived gravity flows (Missoula type) which may directly enter adjacent deep-water basins, thus forming cyclically stacked turbidite systems. These systems and their bearing upon current sequence-stratigraphic models of deep-water siliciclastic sedimentation remain unexplored.