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Several unconformities occur in the Paleozoic and Mesozoic shelf sequences of the Alberta Rocky Mountains. The stratigraphic succession is repeated several times because of thrust faulting. This repetition, combined with excellent exposures, permits a study of the lateral variations in the stratigraphic units and of the contacts of these units. Most of the stratigraphic breaks are disconformities in local outcrops, but regionally some are important angular unconformities.
The stratigraphic, sedimentologic, and paleontologic evidence is reviewed for the following unconformities: (1) Precambrian-Cambrian and Lipalian interval, (2) Cambrian-Ordovician, (3) sub-Devonian, (4) Late Devonian Frasnian-Famennian, (5) Devonian-Mississippian, (6) Carboniferous-Triassic, and (7) Triassic-Jurassic boundary and gaps in the Jurassic sequence.
The important criteria for recognition of these breaks in the stratigraphic succession are, in order of importance: (1) regional stratigraphy, (2) paleontology, and (3) sedimentary phenomena. Of the sedimentary phenomena, eroded surfaces or truncations and residual concentrations of quartz and chert are very useful. Fossils also are useful for locating stratigraphic breaks. Other features, including phosphates and abrupt changes of lithology, also are associated with some unconformities. In several cases it is impossible without paleontologic evidence to determine the position of a particular stratigraphic break even with complete exposure and closely spaced stratigraphic sections.
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