Sedimentology of the Penrith sandstone and brockrams (Permo-Triassic) of Cumbria, north-west England
The Penrith Sandstone and brockrams (breccio-conglomerates) are continental red-beds of Lower Permian and Permo-Triassic ages respectively. The brockrams are predominantly the deposits of coalescing alluvial fans which accumulated as the products of subaerial sheetflood, transitional debris flow and braided river processes at the margins of desert basins bordering the upstanding Lake District. The fans were deposited on an irregular topography which is demonstrated to have a probable minimum preburial relief of 250 to 300 metres in west Cumbria. The cross-stratified Penrith Sandstone of the Eden Valley accumulated as the foresets of large scale (up to 100 metres width) aeolian dunes orientated in response to a uniform east-south-easterly palaeowind. Dune-bedding within these deposits indicates a crescentic-downwind (barchanoid) configuration for the dune slipfaces. The stratigraphic unit termed the 'Upper Brockram' is believed to represent the deposits of ephemeral streams (arroyos) which flowed through and reworked part of the aeolian sand sea of the Penrith Sandstone. Nodular carbonate profiles occurring within sandstone/siltstone horizons intercalated with distal alluvial fan deposits in west Cumbria are interpreted as immature caliche soil horizons and represent periods of surface stability (non-deposition) of indeterminate length. Petrographic evidence indicates the most important process of formation to have been that of replacement of the detrital quartz component by microcrystalline non-ferroan calcite. Extensive post-depositional (diagenetic) modification to the mineralogy and texture of the brockrams of west Cumbria has resulted from the mechanical infiltration of clay, the dissolution of the less stable detrital components (primarily volcanic clasts), replacement by secondary clay and the precipitation of interstitial and void-filling authigenic clay, a hydrated haematite precursor mineral, ferroan and non-ferroan calcite and gypsum. Most reactions are considered to have occurred as stages within an evolving diagenetic sequence.