CONSTRUCTIONAL LITHOFACIES GEOMETRY OF A SUBAQUEOUS ESKER DELTA COMPLEX AS A CONTROL ON THE EVOLUTION OF SEDIMENT BODY POROSITY AND PERMEABILITY

High velocity axial jets of glacial meltwater from the Laurentide Ice Sheet deposited an esker and subaqueous delta complex into ancestral proglacial Lake Iroquois. The esker cross cuts the recessional Carlton Moraine and terminates in a subaqueous fan. Multiple subglacial discharge events in the esker tunnel formed a series of stacked, low relief layers of cross bedded, clast supported sandy gravels and matrix supported gravelly sands, amalgamated through episodic bursts of high energy scour and fill.

Gravel beds are often >1m in thickness and may represent longitudinal bars formed by torrential channelized flows. The lateral continuity and succession of reactivation surfaces within the gravel lithofacies indicates migrating gravel waves through a subglacial tunnel system traceable from ~1km offshore to the proximal fan ~2km from the present Lake Ontario shoreline. Extensional faulting occurs along the edge of the gravelly lithofacies and probably formed by tunnel collapse during ice margin wastage.

Tabular gravel lithofacies are draped and prograded by cross bedded and rippled sandy lithofacies. The overall sequence is concluded with a cap of rhythmic bedded sands and silts with occasional dropstone cobbles. Distal fan morphology of finer lithofacies indicates deposition of hyper-concentrated suspended load after rapid decrease of axial jet velocity into standing water. Smaller outwash ridges preceded the central fan as the initial distributary network.

Lithofacies properties, sediment body architecture and constructional landscape morphology influence modern hydrogeologic properties of the esker delta complex. Sediment body architecture was influenced by meltwater processes during subglacial tunnel and proglacial delta sedimentation.

The gravelly lithofacies had initially high permeability that subsequently was reduced by meniscus cementation of meteoric calcite as subglacial fluid pressures relaxed and bedrock buffered groundwaters migrated into the gravels. The overlying sandy and silty lithofacies textures remain too coarse to form a confining layer over the gravels. The overall architectural geometry controls the zonation of porosity and permeability, producing a sediment body with very high permeability, high infiltration capacity but restricted transmissivity due to the limited size of the deposit.