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.
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