LOWER BORDER GROUP (TOURNAISIAN) FLUVIO-DELTAIC SEDIMENTATION AND PALAEOGEOGRAPHY OF THE NORTHUMBERLAND BASIN

Abstract

Summary Lower Border Group (Tournaisian) successions in the Northumberland basin are divided into seven formations. Only broad correlation between laterally equivalent formations is possible owing to facies changes. Facies association 1 comprises thick, erosive-based sandstones of meandering fluviatile and delta-distributary channel origin. Facies association 2 comprises coarsening-upwards shale to sandstone subfacies deposited in lacustrine, pro-delta, delta-front and marginal delta embayment environments. Facies association 3 comprises alternating thinly-bedded sandstone, siltstone, mudstone and carbonate subfacies deposited in interior fluviatile flood basins, coastal plain flood basins and playa-lakes and on-delta backswamps. Palaeocurrent evidence indicates river and delta influxes from the north-west and the north-east. The Whita fluvio-deltaic system drained from Lower Palaeozoic sedimentary source-lands in the Southern Uplands to the north and north-west. Clastic input from this system had almost ceased by the end of Tournaisian times. The Bewcastle deltaic system advanced along the axis of the Northumberland basin from the north-east during the later Tournaisian. Each delta advance (thirteen in all) closed off and then infilled a shallow embayment, the Newcastleton embayment, located north of the axis of active delta-front sedimentation. Advance of both delta systems into the basin was periodic, with gulf carbonate facies deposited during periods of delta absence. Detailed study of the facies deposited by a single advance of the Bewcastle delta system indicates: (1) delta advance into relatively shallow waters (2) development of delta-front sheet sands and interdistributary bay deposits (3) temporary abandonment of a portion of the delta-front and (4) absence of tidal and wave current modifications to the advancing and retreating delta-fronts. During Lower Border Group tune the Northumberland basin was bounded by land areas to the north and south. The Whita and other fluvial systems deposited thick alluvial wedges of sediment at points along the northern basin margin. These indicate a general southerly palaeoslope over much of the Southern Uplands area.