Facies, architecture, and bed-thickness structure of turbidite systems: Examples from the East Carpathian Flysch, Romania, and the Great Valley Group, California

This report explores the sedimentology, facies architecture, and bed-thickness structure of turbidite systems. Turbidite successions tend to have either a power-law or an exponential bed-thickness distribution. In the out-of-channel settings of the Amazon Fan, the B exponent of bed-thickness distributions tends to decrease, whereas facies clustering, as measured by Hurst H, tends to increase in a downfan direction. Flow filtering refers to the combined process of sand-mud fractionation and flow-volume modification through overbank flow that characterizes the construction of large levees by stratified flows. A wide range of flow sizes and a skewed flow-volume distribution are necessary conditions for development of Amazon-type channel-levee systems.; Facies- and architectural element analysis were used in the study of thick-bedded deep-water sands of two formations. The Paleogene Tarcau Sandstone of East Carpathians, Romania, is a more than 900 m thick succession that reflects progradation of a large submarine fan, with levee-, crevasse-splay-, and proximal channel deposits overlying distal channel- and proximal lobe sediments. Pebbly sandstone packets have a well-developed clustering and symmetric grain-size- and bed-thickness trends.; A seismic-scale onlap surface probably exists in the Turonian Venado Sandstone of the Great Valley Group, northern California, against which at least five sand-rich packets pinch out. This geometry results in an apparent progradational stacking pattern. In contrast, a more southern section is coarser grained, thicker bedded and has an overall retrogradational stacking pattern; the base of the Venado is an erosional conglomerate-mudstone contact. It is likely that uplift along the southwestern margin of the basin influenced the deposition of the Venado. Dense sampling of mud-rich and mud-poor thick-bedded deep-water sands of Oligocene age from the East Carpathian Flysch shows that coarse-tail normal grading is associated with fine-tail inverse grading. The latter is the result of an increasing effectiveness of winnowing during deposition from a collapsing turbidity current, as the initially high suspended-load fallout rate declines. Thus, thick inversely graded intervals in deep-water sands lacking traction structures do not necessarily imply waxing flow. Quantitative textural studies are useful in distinguishing between deposits of collapsing vs. sustained currents and turbulent vs. laminar flows.