Miocene lake basin analysis and comparative taphonomy: Clarkia (Idaho, U.S.A.) and Shanwang (Shandong, P.R. China)

Lithofacies and taphofacies of the two Miocene lake basins, the Clarkia basin in northern Idaho, U.S.A., and the Shanwang basin in Shandong Province, P.R. China, are documented. Sedimentologic and paleoecologic data from both outcrops and drill cores, coupled with geophysical and geochemical studies, provide new information on the sedimentary history of the lake basins and depositional, taphonomic, paleoclimatic, and paleolimnologic controls of the two extraordinarily preserved Miocene fossil biotas. Analysis and comparison of taphonomic processes and conditions of such diverse soft-tissued fossil biotas, with similar age, paleoclimatic and paleoenvironmental conditions, demonstrate the favorable conditions for the preservation of extraordinary fossil biotas in lacustrine deposits.; The comparisons provide the following new interpretations for the two lake basins: (1) Both lake basins were formed in relation to Tertiary flood basalts. The Clarkia lake was created by Columbia River basalt damming the proto-St. Maries River valley, but the Shanwang lake was the result of water accumulation in a depression above the Niushan Formation basalt lavas. (2) The two Miocene lakes may have been similar in water chemistry and high sedimentary rate but differed somewhat in configuration, depth, thermal stratification, and trophic condition. (3) Humid warm-temperate paleoclimate is indicated for both areas during Miocene time, with Clarkia being more humid, warmer, and less seasonal than Shanwang. (4) Taphofacies of the two soft-tissued fossil biotas show overall similarity with high original coloration and low breakage, abrasion, bioerosion, and orientation; but they can be distinguished by slightly different taphonomic signatures of articulation, ultrastructure, compaction, and ecological spectrum. (5) A special combination of unusual mass-killing (or unseasonal shedding) of organisms provided fresh organic materials directly transported over short distances; and rapid burial in anoxic and low temperature lake-bottom water produced extraordinary fossil biotas with less diagenesis in both lake deposits. (6) Reevaluation of previously-studied fossil localities and prediction of new occurrences of such well-preserved fossil biotas in lacustrine settings will be advanced by understanding the favorable physical, chemical, and biological parameters that enhance the formation and preservation of soft-tissued fossil biotas (fossil lagerstatten).