Abstract

Continuous brine-trench exposures of Recent evaporitic sediments, cut through the top few meters of the Qaidam Basin, western China, record the temporal evolution of saline-lake to saline-lake-margin subenvironments; correlation of stratigraphic sections from different parts of the depocenter record the spatial evolution of these subenvironments. Shallow perennial saline lakes cycle through various stages (dilute lake, saline lake, saline pan, and desiccated pan) and this produces characteristic suites of sedimentary textures and structures. In addition, main-lake and lake-margin subenvironments contain quite different suites of sedimentary textures and structures. Recent sediments from the Qarhan Salt Plain consist of laminated siliciclastic muds (13-50 cm thick, devoid of mudcracks) overlain by halite (centimeter-scale) and interlayered mud (submillimeter- to centimeter-scale), termed mud-halite couplets, which represent one shallowing- and concentrating-upwards succession. Sediments of shallow perennial lakes consist of a basal laminated mud overlain by mud-halite couplets with dominantly conformable contacts. Halite layers are crystalline frameworks made up of halite cubes and chevrons. Mud-halite couplets with ubiquitous dissolution surfaces and heavily dissolution-modified halites make up saline-pan sediments and subaerially modified halites, respectively. This succession of sediments records the flooding and desiccation of a 200 km 2 lake with waters depths of 2.2-3.0 m. The lateral continuity of beds and the three-dimensional array of facies show that this lake was filled by a combination of progradation and basin-wide aggradation. Mud-rich and mud-poor packages can be traced for several kilometers in the center of the Holocene lake, whereas units are less laterally continuous on the margins of the lake, near the inflow zone. Syndepositional dissolution pipes in halite layers suggest subaerial modification and are useful in tracking lake expansion and contraction through time. Vertical successions of structures and textures distinguish deposits of shallow perennial saline lakes from deposits of ephemeral salt pans in the geologic record. Sediments deposited in shallow, nonstratified, perennial saline lakes may be host to one or more of the following: laterally continuous layers; muds undisrupted by subaerial exposure; erosional channels; cumulate halite; vertically bottom-grown halite; halite layers conformably draped by mud; and halite layers truncated by nonuniformly spaced dissolution surfaces.

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