Giant lacustrine bioherms in the northeastern Qinghai-Tibet Plateau (western Qaidam Basin) during the Early–Middle Miocene: Carbonate fabrics, growth patterns, and environmental significance

Abstract

The development of lacustrine carbonate bioherms in the Qaidam Basin provides critical insights into the influence of the Qinghai-Tibetan Plateau (QTP) uplift and climate change on Asian interior lake ecosystems. Two distinct, successively developed phases of carbonate bioherm proliferation, each extending laterally for over 3 km, have been identified within the Lower to Middle Miocene siliciclastic-dominated strata of the western Qaidam Basin, northeastern Tibetan Plateau. These bioherms exhibit diverse morphologies—ranging from large domical structures (>3 m high) to smaller hemispherical and inverted conical forms (<1 m)—and are arranged either as biostromes within a single stratigraphic level or in complex, vertically stacked patterns. Green algae (Cladophorites and problematic Chlorellopsis) and putative calcimicrobes were the primary bioherm builders. The dominant alga Cladophorites primarily formed radially branching and dense-reticular frameworks. Putative calcified filamentous and spherical microbes, acting alone or in association with green algae, formed foundational cores or substrates. These were subsequently encased by stromatolitic crusts, which aggraded iteratively from lithified microbial mats via microbially induced mineralization. The frameworks contain abundant microsparitic (4–30 μm) calcite crusts and laminated fibrous calcite cements, complemented by fringing dolomite cement crusts at the margins; these features collectively enhanced the lithification and preservation of the bioherm structures. Through multi-scale petrographic and sedimentological analysis, ten distinct lithofacies were identified within the bioherms and adjacent strata. Based on the spatial and temporal distribution of these lithofacies, a three-stage evolutionary model is proposed for the bioherms, which includes: (1) an initiation stage on littoral quartz sand bars and ooid/peloid shoals; (2) a development stage during a gradual transgression; and (3) a termination stage marked by the deposition of littoral muds or distal sublittoral silts. The proliferation of large, algal-microbial bioherms in the plateau-type Qaidam Basin required a specific set of environmental conditions: warm, fresh-to-brackish waters, sufficient accommodation space, high alkalinity, and abundant nutrients. We propose a model highlighting the integrated effects of an enhanced hydrological cycle across the QTP, catalyzed by the Miocene Climatic Optimum. The spread of these carbonate bioherms into the typically cold, arid, and saline lacustrine environments of the QTP serves as a significant indicator of this extreme warming event, providing valuable insights into potential biological and sedimentological feedback mechanisms under analogous present-day warming conditions.