Anatomy, hydrodynamics and depositional setting of a Westphalian C lacustrine delta complex, West Midlands, England



Studies, spanning a 3 year period, of Westphalian C strata exposed in an active quarry have enabled three dimensional reconstruction of a lacustrine delta complex. The sequence exhibits a complex history of lake infilling by sediment introduced by intermittent high energy, low sinuosity distributary channel flows.

Deposition in the small 0.2 km2 lake prior to delta formation was dominated by organic matter and typified rheotrophic swamp conditions. Large lycopods colonized the swamp floor. The lake was filled by a prograding delta which comprised six horizontally and vertically stacked delta lobes. The presence of lycopods aided sediment accumulation.

Fluctuations in river discharge and consequent lake level rise and fall exerted a fundamental control on delta progradation and aggradation. Flooding during low lake levels first produced erosion on the existing lobe followed by a lake level rise which created accommodation for aggradation of a new delta lobe. Further lobe erosion and low stand lobe formation occurred during lake level fall.

The area's proximity to alluvial fans resulted in hyperconcentrated flood flow within the distributary channels. The occurrence upon in-channel surfaces of plant colonization, including lycopods, testifies to the ephemeral nature of the flow. The lacustrine delta complex formed at the front of a terminal alluvial fan. Northward progradation of the alluvial fan was achieved by the capture and infilling of lakes by northerly flowing distributary channels.