Cosmogenic 10Be–26Al Isochron Burial Dating of Basalt-Capped Fluvial Deposits in the Hantan River Basin, Korea

The Hantan River basin (central–northern Korean Peninsula) is characterized by a Quaternary volcanic–fluvial landscape in which basaltic lava flows repeatedly overlie unconsolidated fluvial deposits. Beneath these lava flows, the Baeguiri Formation consists of channelized fluvial conglomerates and sands with associated overbank silts and clays, unconformably overlying Devonian basement units. While the emplacement ages of overlying basalts are relatively well constrained by K–Ar and ⁴⁰Ar/³⁹Ar dating, the depositional and burial timing of the underlying fluvial conglomerates remains poorly constrained and controversial. Previous age estimates for the Baeguiri Formation, derived indirectly from basalt contact ages, fission-track, and TL/OSL methods, provide only partial constraints and may be affected by inheritance, post-depositional reworking, and uncertainty in the time lag between sediment deposition and lava burial.

To directly date the burial of the fluvial conglomerates and to assess potential temporal heterogeneity among key outcrops, we apply cosmogenic radionuclide (CRN) burial dating using paired ¹⁰Be and ²⁶Al measurements on quartz-bearing gravel from conglomeratic units of the Baeguiri Formation at four representative sites in the Hantan River system: Eundaeri (Chatan Stream tributary), Baeguigyo Bridge, Jangjingyo Bridge, and Hantangang Dam along the mainstem Hantan River. At all sites, the conglomerates are fully covered by multi-meter-thick basalt flows, locally associated with pillow lavas and slabby flow-top structures, indicating rapid burial and long-term attenuation of cosmic-ray exposure. Burial ages are calculated using isochron approaches to account for variable pre-burial exposure histories and possible partial reworking within fluvial deposits.

The resulting burial ages provide direct chronological constraints on the timing of isolation of the Baeguiri Formation from surface processes, allowing us to (1) Test whether burial of the fluvial conglomerates by basalt flows was synchronous across different outcrops, (2) Quantify spatial and temporal heterogeneity in sedimentation–burial events within the Baeguiri Formation, and (3) Evaluate implications for refining stratigraphic relationships between fluvial deposits and multi-stage basalt emplacement in the Hantan River volcanic–fluvial succession. This study highlights the effectiveness of CRN burial dating for directly constraining the age of basalt-buried fluvial conglomerates in complex Quaternary volcanic plateaus.