Abstract: Sequence Stratigraphy: Fact, Fantasy, or Work in Progress
D. A. Leckie, L. F. Krystinik
Sequence stratigraphy has been hailed as a magic elixir to cure exploration problems in mature and frontier basins. Yet, like most cure-alls, analyses of modern depositional systems show that critical assumptions regarding sequence stratigraphy merit further review. An example from the modern Canterbury Plains, New Zealand, demonstrates some of the potential pitfalls of sequence stratigraphy and its application to hydrocarbon exploration.
The Canterbury Plains, New Zealand, bounded by the Southern Alps and Pacific Ocean, are 60 km wide and 185 km long, and traversed by four large gravel rivers. The Canterbury basin is up to 750 m deep, filled primarily with gravel. The coastline is wave dominated and microtidal, with high rates of north-directed longshore drift. The southern coast is transgressive with 22 m wave-cut cliffs in Pleistocene gravel. Beaches are gravel and sand. Coastal erosion at approximately 1 m/yr steepens the fluvial gradient, causing the rivers to incise 1.5-4.2 mm/yr during the present highstand. Kilometer-wide river mouths have incised into the regional flood plain, with the amount of incision decreasing inland. There are only minimal estuarine conditions at the mouths of the incised valleys. River eadwaters in the Southern Alps are uplifting, tectonically causing incision, which decreases seaward. Thus, fluvial incision takes place in the west due to mountain uplift and in the east due to the transgressing shoreline. A zone of null valley incision occurs 8-15 km from the coast. Existing sequence stratigraphic models suggest that downcutting should occur during falling sea level, not during transgression.
The southern coastline is separated from the northern coastline by Banks Peninsula, a resistant volcanic complex that acts as a large groyne to southerly waves. The northern coastline progrades approximately 1 m/yr and is largely sandy. Thus, the coastline within the same basin during the present sea level highstand is at one locale progradational and elsewhere transgressive. Gravel reaches the transgressive coast, where a steep gradient is maintained by downcutting. The Canterbury cliffs have implications for the depth of erosion during a marine transgression. The thickness of material eroded during transgression, including last-cycle, late glacial to Holocene cliffs, is approximately 40 m. Dominant controls on sedimentation are the high-wave-energy coastline, highly variable but ext eme rainfall, rising mountains, and the subsiding basin. Although sea level plays a role, more important controls on progradation, transgression, and valley incision are the extreme wave energy and longshore drift. The east coast of South Island also provides insight into the problem of "predicted" missing lowstand shorelines. Where do they go and why are they not always at the mouths of incised valleys?
This example provides insight into the potential risks of making broad extrapolations from local observations. Essentially, beware the model; look at the rocks.
AAPG Search and Discovery Article #90953©1995-1996 AAPG Distinguished Lecturers