M. T. Whalen, G.P. Eberli, F.S.P Van Buchem, E.W. Mountjoy, P. Homewood
Sequence stratigraphic analysis of the southeast margins of two isolated Late Devonian carbonate platforms (Miette and Ancient Wall) facilitated refined correlations between platform and basin filling successions. The key to these correlations involved identification and lateral tracing of lowstand basinal deposits, including coarse carbonate debris flows and fine-grained siliciclastics, that onlap sequence boundaries. In general transgressive systems tracts (TSTs) are characterized by an abrupt deepening in shallow platform and basinal environments. Highstand systems tracts (HSTs) consist of prograding or aggrading shallow-water carbonates and stromatoporoid framestones and basinal calcareous turbidites and mudstones. Lowstand system tracts (LSTs) are represented by carbonate debris flows , shales, and shaly limestones in basinal and slope settings and unconformities or thin shoaling upward successions on platform tops.
At least ix Late Devonian depositional sequences were deposited from Late Givetian to Famennian time. S-1 (Flume Fm.) represents initial flooding of a subaerial exposure surface (SB-1) and development of a regionally extensive carbonate ramp. Ramp flooding, recorded by the TST of S-2 (Maligne Fm.), is followed by the development of isolated prograding and aggrading carbonate platforms (HST, Cairn Fm.). S-2 is capped by an unconformity with up to 3 m of erosional relief (SB-3). S-3 and 4 are similar in that; 1) basinal LSTs contain sheet-like and/or channelized carbonate debris flows and shales that onlap the underlying sequence boundary , 2) basinal HSTs include redeposited carbonate mudstones and turbidites. The platformal HST of S-3 (lower Southesk Fm. = Peechee) exhibits pronounced backstepping and aggradation while that of S-4 (upper Southesk Fm. = Arcs) is progradational. S-5 (uppermost Southesk Fm. = Simla/Ronde) is composed of prograding carbonate wedges interbedded with silty limestones. The uppermost wedge shoals upward into an erosionally truncated intertidal thrombolitic bafflestone (SB-6). The LST and TST of S-6 (Sassenach Fm.) are composed of quartz-rich carbonates that onlap SB-6 and thicken basinward. Transgression provided accomodation space for the overlying, aggrading shallow water carbonates (HST, Palliser Fm.).
The stacking pattern of this mixed carbonate-siliciclastic system differs from that predicted by sedimentary models developed for pure carbonate or clastic systems. Unlike pure carbonate systems, clastic sediments were deposited during lowstands in basinal areas surrounding Late Devonian isolated platforms. Unlike pure clastic systems, highstand carbonate platform shedding provided a significant component to the Late Devonian basinal sediment. These relationships point out the need for hybrid sedimentary and sequence stratgraphic models to accurately describe mixed carbonate-siliciclastic systems.