THE MECHANISM FOR CRETACEOUS TURBIDITE DEPOSITION IN LOWER PRIAMURYE

The mechanism for deposition of Cretaceous turbidites in Lower Priamurye, just like all turbidites in general, is based on the hydrodynamic regime in the basin of their accumulation. The scheme of the hydrodynamic regime of turbidite sedimentation is given using the Berriasian-Valanginian basin of the Gorinsky terrane as an example. Sediments from the eastern margin of the Eurasian continent accreted at the continental slope to be moved eastward to its base by turbidity currents created by submarine slumping of sediments. The coarse material of the landslides settled out first, and fine-grained particles were carried by turbidity currents further east into the deeper parts of the basin. At the base of the slope, turbidity currents interacted with bottom currents which would circulate at the base of the continental slope as large-scale eddies. Coarse-grained clastic turbidites were deposited from the highly turbulent (high-velocity) flows oriented from west to east. As they moved further east, the turbidity currents weakened and deviated to acquire first a submeridional (from south to north) orientation and then a sublatitudinal (from east to west) orientation. As a result, sandy turbidites were deposited from submeridional turbidity currents, while sandy-clay and silty-clay cyclites from sublatitudinal currents. During periods of the generally diminished tectonic activity in the region when the influence of turbidity currents decreased, only bottom currents circulated in the basin to form narrow horizons of contourites. The area where sandy turbidites accumulated could later on become an arena for the accumulation of sandy-clay and silty-clay turbidites and vice versa. As a result of frequent changes in the hydrodynamic regime, a sedimentary section was formed represented by alternation of different-type turbidites and, to a lesser extent, contourites.