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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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It is commonly possible to decide that an ancient body of rock was laid down in a shallow-water marine environment but in many cases it is impossible to determine precisely which shallow-water marine environment.
The Ordovician Stairway Sandstone, a shallow-water marine formation of the Amadeus basin, central Australia, contains appreciable, though so far non-commercial, quantities of oil, gas, and phosphate. It is therefore desirable to have a detailed knowledge of the depositional environment of this formation. For this reason, extensive field and laboratory studies were undertaken.
These studies have revealed that the sedimentary rocks, mainly orthoquartzite and phosphatic shale, were deposited during a regressive-transgressive cycle which resulted in the migration of a single shallow-marine depositional environment across at least 40,000 square miles of the basin. This has profoundly influenced facies distribution. Using a detailed graphic-log approach, the numerous sedimentation units in the Stairway Sandstone can be grouped into six composite units. These, in turn, comprise a single compound sedimentation unit whose characteristics are the result of a particular depositional environment.
The characteristics of this compound unit, compared with those of modern sediments, show that the modern lagoon-barrier and intertidal-flat sediments
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are very similar in both lithologic character and sequence to those of the compound sedimentation unit of the Stairway Sandstone. However, because of the many thousands of square miles covered by the facies of the Stairway Sandstone, both modern models are considered to be inadequate. Therefore a more hypothetical model of epeiric-sea sedimentation also is considered. Though having the disadvantage that there is no known present-day counterpart, this model nevertheless warrants some consideration. It is possible to explain many of the features of the Stairway Sandstone (and perhaps other formations) by its use.
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