Taphonomy and Diagenesis of Freshwater Bivalves from the Late Albian Dalazi Formation, Yanji Basin, Ne China: Preliminary Data and Paleoenvironmental Implications

The Lower Cretaceous Dalazi Formation records a late Albian lacustrine depositional system that represents a continuation, or post-rift fauna, developed after the main flowering period of the Jehol Biota, containing similar aquatic fossils. Amongst the preserved fossils is an assemblage of freshwater bivalves on which we herein present preliminary results from an integrated taphonomic, paleoecological and diagenetic investigation. The assemblage was recovered from the Zhixin Section, which exposes finely laminated organic-rich siltstones and mudstones of the upper member of the Dalazi Formation, and is dominated by two species: out of 103 specimens analyzed, 50 belong to species Trigonioides helongjiangensis (48.5%) whereas 53 to Sphaerium sp. (51.5%), both biostratigaphically significant for the non-marine Albian of northeastern China. Articulation data reveal that 30 specimens (29%) are articulated: 21 as closed valves (20.4%) and 9 (8.6%) are in butterfly position, while 73 (70.8%) are disarticulated. Preservation categories include internal molds (22.3%, n = 23), external molds (23.0%, n = 24), and composite molds (6.8%, n = 7), with 85.4% (n = 88) of specimens retaining original shell material, some of them nearly completely preserved, others representing shell remains within the aforementioned molds. Shell abrasion and fragmentation are absent or very low. Height-to-length ratios are similar in both species (0.68-0.70) despite marked differences in absolute shell dimensions and Reduced Major Axis regression suggests isometric growth pattern with a narrow range of morphological variability in both taxa. Of 14 specimens screened by Raman spectroscopy, three — two S. sp. And one T. helongjiangensis — display spectra consistent with pristine aragonite (characteristic doublet at 701/705 cm⁻¹ and a strong peak at 1085 cm⁻¹), while the remaining 11 show noisy, poorly defined, patterns indicative of diagenetic overprinting. Optical cathodoluminescence imaging and EDS mapping on two polished cross-sections document: a sharp luminescence boundary between an outer aragonitic zone and an inner calcitic zone tentatively interpreted as a snapshot of an arrested neomorphic replacement front. The preliminary analysis suggests that: (i) the assemblage is autochthonous to parautochthonous, with no clear evidence of hydraulic sorting or significant transport, pointing to in situ or near-in situ accumulation in a low-energy, fine-grained lacustrine bottom; (ii) rapid burial under sustained reducing conditions — consistent with the independently reconstructed anoxic ferruginous deep-water setting of the Dalazi Formation — drove early diagenetic aragonite retention in selected specimens; (iii) the integrated taphonomic and diagenetic record is congruent with a stratified, sulfate-poor paleolake, providing a rare window into non-marine diagenetic processes during the mid-Cretaceous super-greenhouse. These results contribute new data on invertebrate paleoenvironments related to the Jehol Biota and support ongoing non-marine to marine Cretaceous correlation efforts in northeastern Asia.