Abstract

The early Devonian, Old Red Sandstone succession at St. Cyrus, NE Scotland, includes the exceptional preservation of a lava field and its associated fluvial drainage system. The lava field developed through punctuated eruptions of low-volume pāhoehoe lava, fed by lava tubes. During periods of rapid effusion, groundwater systems were able to transmit most of the recharge in the subsurface, probably by springs, and where there were areas of localized low permeability, ephemeral lakes developed. Subsequent lava flows underwent significant interaction with the partly consolidated lacustrine sediment, forming peperite deposits. Fluvial conglomerates, with extra-basinal clasts, are interbedded with the lava flows. The maturity of the fluvial system suggests a long-lived catchment that transported sediment westwards. Drainage was periodically deflected away from the lava field by the positive topographic relief created by the lava flows, and was re-established during periods of volcanic dormancy. The lava field developed over periods of decades, but the fluvial systems were longer-lived by three or four orders of magnitude. A key feature of this lava field and its drainage is that the establishment and abandonment of fluvial systems immediately adjacent to the lava field was primarily controlled by the eruption dynamics of the magmatic system.

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