The mirror of a Late Ordovician post-glacial flooding – a conglomerate beachrock from the Tarim Basin

Understanding ancient climate changes is hampered by the inability to disentangle trends in continental ice volume from records of relative sea-level change. As a unique coastal deposit in tropical and subtropical regions, beachrock has been proved to be reliable for constraining the glacial meltwater signal and, thus, the total volume of land-based ice in the Quaternary. However, beachrock is rarely recognized in the fossil record due to (a) the 2-dimensional distribution of beach deposits, as opposed, for example, to extended platform sediments, and (b) the fact that specific environmental conditions are required in order to lithify sediments directly at the beach.

By combing the stratigraphic architecture with petrography of characteristic cements, we show the first Ordovician beachrock from the Tarim Block, northwestern China. According to biostratigraphic data, a middle Katian (Upper Ordovician) palaeokarst surface is capped by a carbonate conglomerate beachrock, indicating a significant late Katian relative sea-level rise.

These beachrocks can be correlated with the onlap on the widespread subaerial exposure surfaces during deglaciation and post-glacial sea level rise. They formed in northwestern Tarim Basin after a pronounced stratigraphical gap reflecting the expression of a Katian glacial. We suggest that the beachrock ‘fingerprinted’ a strong melt-water pulse in high latitudes after this short-lived glaciation, which until now did not receive much attention in the scientific literature.