Abstract: Turbidite Systems and Their Relations to Catastrophic Fluvial Sedimentation
Turbidite systems record periods of time during which parts of basinal regions become the sites of important sand accumulations originated by submarine sediment gravity flows. These systems have facies characteristics, internal stacking patterns, and types of geometry that vary greatly from one system to another, mainly as a function of the local tectonic setting, the type of sediment available in the source zone, and the transformations experienced by gravity flows during their downslope motion. All systems can essentially be viewed as parts of a continuum ranging from small, short-lived, and internally poorly organized features to large and long-lived features characterized by distinct stages of growth and well-developed hierarchical cyclic stacking patterns.
Although turbidite deposition may occur in all types of basins, the largest accumulations of turbidite sandstones are typically observed in tectonically active basins formed during the various stages of development of thrust-fold belts. Because they are generally unaffected by appreciable bottom-current activity, these basins are the most suitable for the study of ancient turbidite systems and their internal stacking patterns. In such settings and particularly in narrow and elongate troughs associated with thrust propagation, enormous volumes of sand stored in adjacent shelfal and alluvial regions were periodically transferred into highly subsiding and relatively deep basins by gravity flows. How the process occurred and its causes are still poorly understood.
Once initiated, gravity flows undergo downslope transformations that are recorded, within each system considered, by specific facies tracts both laterally and vertically. Careful analysis of genetic facies tracts within the framework of high-resolution stratal correlation patterns delineates the transfer and depositional zones of gravity flows at each specific stratigraphic interval considered. A transfer zone is characterized by predominant sediment bypass; a depositional zone is one where each flow deposits the bulk of its sand load. Typical depositional zones are expressed by parallel-sided sandstone beds that stack to form sheetlike bodies or lobes.
Within each system, the volume of gravity flows determines the positions of the transfer and depositional zones and, consequently, the position of the main sand accumulation at each considered time. The stacking patterns observed in most of the large and long-lived turbidite systems show that systematic shifting of both transfer and depositional zones occurred through time as a result of changes in the volume of gravity flows. Zonal shifts of relatively different importance define a hierarchy of units which, from the smallest to the largest, include elementary facies tracts, facies, facies associations, and stages. At different physical scales, each of these units records a forestepping-backstepping episode of sand deposition commonly expressed by an overall thinning- and fining-upwar trend.
The origin of the cyclic stacking patterns observed within turbidite systems poses the basic problem concerning the origin of cyclic periods of initiation of gravity flows in the source zones. Major zonal shifts are probably controlled by relative sea level variations that determine the distance between river mouths and the basin. This distance increases with rising sea level, thus forcing river-fed sands to be progressively accommodated in shelfal regions. Minor and higher frequency shifts are more difficult to explain in terms of sequence-stratigraphic models.
A growing body of evidence derived from the study of ancient fluvial and turbidite systems indicates that both sedimentological and sequence-stratigraphic models of turbidite systems have overlooked the very close genetic link between fluvial and turbidite sedimentation. Catastrophic floods involving large-volume and high-density mixtures of water and sediment can actually enter adjacent marine basins as sediment gravity flows (hyperpycnal flows). Depending upon their volume and the local physiographic setting, such flows can deposit their sediment load in shelfal or in slope-basinal regions giving way to complex depositional systems termed herein "fluvio-turbidite systems." In these systems and at each considered time, the positions of the transfer and depositional zones of flood-rel ted gravity flows and, consequently, the position of the main sand accumulations are determined by the interaction between relative sea level variations and climatic cycles. The latter exert a fundamental control on the amount of water supplied to each system and therefore on the volume of flood-generated gravity flows.
Sediment failure along basin margins produced by high rates of sedimentation and/or structural deformation as well as earthquake-derived catastrophic flows may contribute additional volumes of sediment to each system.
AAPG Search and Discovery Article #90949©1995-1996 AAPG International Distinguished Lecturers