--> Sequence Stratigraphy I: Fact, Fantasy, or Work in Progress(?) Or, When is an Incised Valley Not a Sequence Boundary?, by D. A. Leckie and L. F. Krystinik; #90986 (1994).

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Abstract: Sequence Stratigraphy I: Fact, Fantasy, or Work in Progress(?) Or, When is an Incised Valley Not a Sequence Boundary?

Dale A. Leckie, L. F. Krystinik

Sequence stratigraphy is hailed as a magic elixir to cure exploration problems. Yet, like most cure-alls, analysis of modern and ancient depositional systems show that critical assumptions regarding sequence stratigraphy merit further review. Depositional systems are subject to highly variable processes which produce non-unique geological solutions. The Canterbury Plains, New Zealand, demonstrates some of the potential pitfalls of sequence stratigraphy. The Canterbury Plains are traversed by four large gravel rivers with a wave-dominated and microtidal coast having powerful longshore drift. The south coast is retrogradational with 22-m wave-cut Pleistocene cliffs. One meter/year coastal erosion steepens the fluvial gradient, causing the rivers to incise 1.5-4.2 mm/yr during the presen highstand. This incision decreases inland. River headwaters are uplifting tectonically, causing incision which decreases seawards. Thus, fluvial incision takes place in the west due to

mountain uplift and in the east due to coastal recession. A zone of null valley incision occurs 8-15 km from the coast. Sequence stratigraphic models suggest that downcutting should occur during falling sea level, not during transgression. The northern coastline progrades approx. 1 m/yr and is largely sandy. Thus, the coastline within the same basin is at one locale progradational and elsewhere retrogradational. Gravel reaches the transgressive coast where a steep gradient is maintained by downcutting. Forty meters of sediment is being eroded during the transgression. Dominant controls on sedimentation are the high-wave energy coastline, highly variable but extreme rainfall, rising mountains, and the subsiding basin. The role of sea level is minimal.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994