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Abstract:

To investigate whether clay minerals reflect periodic variations in paleolake conditions, major-element chemical and X-ray diffraction analyses were done on ultrafine (<0.1 μm) fractions of Pleistocene-Holocene lacustrine sediments from the Double Lakes Formation, southern High Plains, Lynn County, Texas. Above a Cretaceous marine-shale substrate dominated by well-crystallized aluminous dioctahedral smectite, the clay-mineral assemblages of the younger lacustrine sediments show cyclic changes perhaps related to fluctuations in hydrologic environment and climate. The cycles apparently represent evaporatively induced salinity shifts from brackish to saline (perennial), or ephemeral-lake (playa) conditions, as inferred from the predominance of sepiolite, interstratified Mg-smectite, and palygorskite respectively. Two of at least four cycles show a complete sequence from low to high lake level and return to playa conditions. Between sediment units clearly dominated by a single clay-mineral species are intervals containing variable mixtures of chain-structure clays and smectite indicating change in the conditions of paleolake deposition. Chemically, the highest ratios of Si4+ to Al3+ are associated with sepiolite and the highest Mg2+ to Al3+ values are correlated with the abundance of interstratified smectite. Pulses of detritus associated with overland runoff and wind are indicated by high concentrations of illite and minor kaolinite, as evidenced by high K+ and Al3+ contents. Spring deposits (tufas) along a principal drainageway near the lake suggest inflow from groundwater has had a major effect on the nature of the clays.

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