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The Nature of Depositional and Seismic Sequence Boundaries in Cretaceous-Tertiary Strata of the Beaufort-Mackenzie Basin
At least 12 km of Upper Cretaceous to Holocene sediments, and an unknown thickness of older strata, underlie the Canadian Beaufort shelf. Lower Cretaceous and older strata have been penetrated on the landward edge of the Beaufort Sea and outcrop extensively in northern Yukon and adjacent Northwest Territories. Conventional lithostratigraphic divisions have been identified for most of the Phanerozoic succession but the offshore Late Cretaceous to Holocene succession is less conducive to subdivision into formations. The bulk of our knowledge of the offshore geology comes from reflection seismic data, supplemented by about 200 exploration boreholes. Consequently, seismic stratigraphy has been the most useful method of analysis, applying some of the principles of depositional sequence analysis.
Various types of surfaces have been identified — unconformities and hiatal surfaces (surfaces of maximum flooding) being the two most prominent surfaces. In the depositional sequence concept, unconformities, or their correlative basinward conformities, are the bounding surfaces of sequences. There are some practical problems with identifying unconformities and their equivalent conformable surfaces in marine successions, in fact, the most prominent and readily identifiable surface is the hiatal surface, in both well logs and outcrop. The position of an erosional unconformity in a structurally conformable succession can be a subjective choice when using wireline logs, whereas the hiatal surface is commonly more obvious.
Using a modified sequence analysis approach (incorporating ideas from Frazier, 1974, and Vail et al., 1977), the Upper Cretaceous to Holocene succession in the Beaufort-Mackenzie Basin can be divided into at least 11 major depositional sequences. Pre-Upper Cretaceous strata are deeply buried in most parts of the basin and are beyond the resolution of conventional reflection seismic mapping.
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