CRITERIA FOR THE RECOGNITION OF SHALLOW-PERENNIAL-SALINE-LAKE HALITESBASED ON RECENT SEDIMENTS FROM THE QAIDAM BASIN, WESTERN CHINA

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 subenviron ments; correlation of stratigraphic sections from different parts of t he depocenter record the spatial evolution of these subenvironments, S hadow perennial saline lakes cycle through various stages (dilute lake , saline lake, saline pan, and desiccated pan) and this produces chara cteristic suites of sedimentary textures and structures. In addition, main-lake and lake-margin subenvironments contain quite different suit es of sedimentary textures and structures. Recent sediments from the Q arhan Salt Plain consist of laminated siliciclastic muds (13-50 cm thi ck, devoid of mudcracks) overlain by halite (centimeter-scale) and int erlayered mud (submillimeter- to centimeter-scale), termed mud-halite couplets, which represent one shallowing- and concentrating-upwards su ccession. Sediments of shallow perennial lakes consist of a basal lami nated mud overlain by mud-halite couplets with dominantly conformable contacts. Halite layers are crystalline frameworks made up of halite c ubes and chevrons. Mudhalite couplets with ubiquitous dissolution surf aces and heavily dissolution modified halites make up saline-pan sedim ents and subaerially modified halites, respectively, This succession o f sediments records the flooding and desiccation of a 200 km(2) lake w ith waters depths of 2.2 3.0 m. The lateral continuity of beds and the three-dimensional array of facies shelf 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 t he margins of the lake, near the inflow zone. Syndepositional dissolut ion pipes in halite layers suggest subaerial modification and are usef ul in tracking lake expansion and contraction through time. Vertical s uccessions of structures and textures distinguish deposits of shadow p erennial saline lakes from deposits of ephemeral salt pans in the geol ogic record. Sediments deposited in shallow, nonstratified, perennial saline lakes may be host to one or more of the following: laterally co ntinuous layers; muds undisrupted by subaerial exposure; erosional cha nnels; cumulate halite; vertically bottom-grown halite; halite layers conformably draped by mud; and halite layers truncated by nonuniformly spaced dissolution surfaces.