FACIES MODEL FOR A PROGRADING SANDY DELTA IN AN ICE-CONTACT LAKE WITH A RETREATING ICE MARGIN: A RE-INTERPRETATION OF GLACIOGENIC SEDIMENTS AT HAWK CLIFF, LAKE ERIE BLUFFS (ONTARIO, CANADA)

Progradation of deltas in glacial lakes primarily occurs under conditions of normal regression (aggradation of the delta plain and basinward advance of the delta front) or forced regression (during ice margin retreat, associated with fluvial erosion); however, the sedimentary signature of forced-regressive prograding ice-contact deltas, or systems controlled by normal and forced regression, is not well understood.

The northern shore of Lake Erie (Ontario, Canada) contains lake bluffs (up to 45 m above lake level) that expose the ‘Port Stanley Drift’ (diamicts, sands, silts, and clays). Previous studies have interpreted that large deltas were deposited by braided systems (under normal-regressive conditions) following deposition of the ‘drift’. Re-examination of the bluffs sediments, in this study, indicates that a combination of normal- and forced-regression was likely involved in the emplacement of the deltaic deposits. The objectives of the study are to record the suite of facies types within the bluffs sediments, understand the depositional conditions in which the sediments were deposited, and provide a facies model for a normal- and forced-regressive prograding sandy delta in an ice-contact lake.

Seven sedimentary logs were recorded from outcrop sections along the bluffs. Facies types were identified, including diamicts (massive matrix-supported and clast-supported), muds and very fine-grained sands (massive, deformed, stratified, laminated, rippled), and fine- to medium-grained sands (rippled, horizontally laminated, massive, deformed). Genetically-related facies were grouped into four main facies associations (FAs), which record lake bottom (FA1), prodelta (FA2), delta slope (FA3), and delta plain (FA4) sediments. Analysis of the vertical and lateral relationship of FAs, and the nature of facies contacts, formed the basis for the re-interpretation.

We propose that the sedimentary succession exposed at the bluffs in the study area contains a nearly-continuous depositional record of a prograding delta in an ice-contact lake under normal-regressive conditions (FA1, FA2) which switches to a forced-regressive prograding sandy delta (FA3, FA4). The results of this study may serve as a facies model for ice-contact sandy deltas under both normal and forced-regressive conditions.