CONCEPTUAL MODELS FOR GLACIODELTAIC DEPOSITS, THE PRESENT PARADIGM, AND LESSONS TO BE LEARNED

Coarse-grained deltas are major parts of the stratigraphic paradigm for mapped Pleistocene lake deposits. Glaciodeltaic deposits are categorized as morphosequences and are chief elements of a revised classification for basin architecture. Stratigraphic and sedimentologic examples and concepts have established rules that must now be used to construct realistic subsurface sections and to devise predictive 3D basin models. Two types of deltas are envisioned: 1) progradational deltas at basin borders, fed from tributary plains or from ice-marginal streams that enter the basin at lake level, 2) progradational and/or aggradational deltas within lakes, which develop in front of, and atop core lacustrine fans or fan/foreset deposits in feeder ice channels. Mappable textural foreset facies show that foreset strata coarsen upward and fine distally from gravel to silty sand facies within uniquely bounded deltaic domains. Proximal sand and gravel facies fills a trapezoidal domain defined by the delta ice-contact slope (20^o-35^o), erosional top contact, and the steep, long foreslope (30^o-35^o) that extends nearly to the bottom of the basin. Most-proximal gravel foresets are in a tall triangular wedge, bounded by the ice-collapse slope, their foreslopes (27^o->36^o), and the basin floor. Gravel forms avalanche/grain-flow cones below lake level where source streams directly enter the lake. To the front, sand and gravel foresets are contained within an inverted trapezoidal domain beneath a fluvial distributary plain. Lowest beds are gravelly toesets, composed of slump/debris-flow/turbidity-flow strata. Sandy foresets form the bulk of the delta in another trapezoid, composed of turbidity-current/lower avalanche/debris-flow deposits, including characteristic ripples and other bed forms. Thin sets of avalanche foresets underlie shallow, convex lobes of the delta-front plain. Distributary streams enter the lake at interlobate concavities and continue as turbid flows down low-sloping, erosional chutes/channels. These channel deposits pass tangentially into lower foresets that aggrade in front of the delta. The gravel topset fluvial wedge domain is disconformable on the older sandy foresets; the delta bottomset domain records continuous aggradation.