Lithofacies and sequence development on an Upper Devonian mixed carbonate‐siliciclastic fore‐reef slope, Canning Basin, Western Australia

Abstract

ABSTRACTDetailed sedimentological and stratigraphical analysis coupled with conodont biostratigraphy of a fore‐reef slope succession in the Napier Range (Napier Formation) is used to develop a depositional model and relative sea‐level history for late Frasnian to late Famennian reef evolution in the Canning Basin of north‐western Australia. Changes in sedimentary style on the slope, reflecting differing rates of carbonate production on the platform, are linked to third‐ and higher order relative sea‐level fluctuations.Overlapping slope aprons accumulated along the base of a steep‐walled platform margin. Coarse carbonate debris was deposited adjacent to the margin as talus breccias (via rockfall) and debris‐flow breccias. Depositional slopes up to 45°, and locally steeper, are demonstrated using rotated geopetal cavity fills. The predominance of channel‐filling lithofacies throughout the slope succession indicates the highly channelized nature of the aprons. The middle slope is dominated by sandy oolitic‐peloidal turbiditic grainstones interpreted as sediment exported from an active platform. The turbidites and associated debris‐flow breccias contrast with condensed carbonate intervals and deep‐water, non‐fenestral stromatolites that record times of very low platform production. Lower slope turbidites and associated intraclastic breccias indicate widespread redeposition of sediment eroded from lithified and semi‐lithified limestones higher up the slope.Several third‐order sequences are recognized in the fore‐reef succession and these are composed primarily of transgressive and highstand deposits. Carbonate production was severely restricted in the early Famennian coinciding with development of onlapping siliciclastic aprons during a relative sea‐level lowstand. Evidence for a subaerial exposure event is also preserved within the siliciclastic strata. Controls on sequence development are difficult to constrain. Although two sequence boundaries can be correlated with falls on the global sea‐level curve, the reef complexes evolved in an active extensional regime and it is highly likely that tectonism, in conjunction with eustasy, controlled accommodation on the platform and therefore carbonate productivity.