252-5 Ordovician Beachrock and Sea-Level Rise: Tracing a Katian Meltwater Pulse in Northwestern China

Session: Climate Transitions in the Paleozoic

Presenting Author:

Authors:

Abstract:

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 demonstrated to be a reliable archive for constraining glacial meltwater signals and, consequently, the total volume of land-based ice during the Quaternary. However, beachrock is rarely recognized in the deep-time fossil record due to (a) its typically narrow and discontinuous two-dimensional distribution, as opposed to extensive shallow-marine platform deposits, and (b) the specific environmental conditions required for early lithification directly on the beach.

Here, by integrating the stratigraphic architecture with petrographic evidence of characteristic meteoric and marine phreatic cements, we report what we interpret as the first known Ordovician beachrock from the Tarim Block, northwestern China. The studied succession reveals a palaeokarstic surface assigned to the middle Katian (Upper Ordovician) based on biostratigraphy, overlain by a well-cemented carbonate conglomerate. This conglomerate includes locally derived clasts with encrusting cements and is interpreted as beachrock formed during a high-energy transgressive event.

Crucially, direct sedimentological evidence observed at the outcrop includes a well-developed erosional marine terrace (wave-cut platform) and associated notches (wave-cut grooves) formed by wave abrasion—diagnostic features of high-energy coastal settings and typical of shoreline stabilization during sea-level stillstands. These features provide compelling geomorphological evidence of subaerial exposure followed by a rapid relative sea-level rise, facilitating beachrock formation through early cementation in the intertidal zone.

The temporal context of this beachrock coincides with widespread subaerial exposure surfaces and a regionally developed stratigraphic gap within the Katian succession of northwestern Tarim. We suggest that this beachrock records a significant meltwater pulse likely linked to the decay of ephemeral continental ice sheets after a short-lived Katian glaciation—a climatic signal that has received little attention in previous studies. Our findings not only document a rare pre-Quaternary example of beachrock but also provide new constraints on Late Ordovician glacio-eustatic dynamics.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8068

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