Sequence architecture, slope-break development and geological significance during the P–T transition in the Mahu Sag, Junggar Basin, China

Abstract

During the P–T transition period (from the Late Permian to the Early Triassic), the Mahu Sag of the Junggar Basin transitioned from a foreland basin to a large depression lake basin, with the development of two sets of unconformities (T1/P and P3w/P2w) and two sets of lowstand deposits, the upper Wuerhe Formation (P3w) and the Baikouquan Formation (T1b) that now host a series of large or giant conglomerate oil fields. For the two third-order sequence boundaries T1/P and P3w/P2w, two types of slope-break zones associated with flexure or erosion have been identified: the flexural slope-break zone controlled by a persistent paleo-uplift that also controls the layer-by-layer “unidirectional onlapping” of strata above the sequence boundary towards the higher part of the slope; and the erosional slope-break zone controlled by the erosional remnants of paleo-mounds, typically characterized by “two-way onlapping” towards the residual mounds or filling along the lower part of the valleys. Both types of slope-break zones control stratigraphy and sedimentary changes, and they encourage the formation of lithostratigraphic traps of up-dip onlapping, lowstand filling, and truncated types. The findings have a significant impact on the exploration of large-scale lithostratigraphic plays controlled by paleotopography.