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In undisturbed depositional sequences one rarely, if ever, observes lateral change directly from mud-supported carbonate
Synthetic seismograms after normal moveout correction for a hypothetical 20-m (66 ft) high impedance sand reservoir in shale. Ratio of compressional velocity to shear velocity in this model is 1.8 for shale, 1.57 for brine sand, and 1.5 for gas sand. Variation in reflectivity with offset in this model is a first order effect over normal recording range.
into grain-supported siltstones, sandstones, or conglomerates. Shale or claystone always intervene. Such bounding and internal contacts are the most conspicuous and most informative--yet most neglected--aspects of the limestone-shale record. Significant progress in understanding carbonate to terrigenous detritus facies changes can come from closer attention to these contacts.
A majority of Mid-Continent Pennsylvanian limestone-shale contacts are of regional extent and commonly are represented by upward gradations from shale to limestone. Shale units in carbonate sections commonly range from fractions of inches to tens of feet in thickness. Genesis of thin (2 ft ot .6 m or less) shale breaks and argillaceous partings has been neglected. This is a serious oversight since such breaks are the connecting links between contemporaneous-land- and shallow-inland-sea-derived sediment.
The following conclusions result from the study of numerous limestone-shale contacts in outcrops and conventional cores in the period 1957 to 1981. (1) The history of Mid-Continent Pennsylvanian sedimentation, including river mouth shifts and numerous floods, is recorded in limestone-shale contacts. (2) The day-to-day regime was one of minor terrigenous sediment bypassing laterally adjacent embayments in which carbonate sediment accumulated. (3) Day-to-day deltaic progradation was minor with carbonate sedimentation curtailed only along the narrow junction between deltas and adjacent embayments. (4) During and immediately after floods, major deltaic progradation spread relatively thin and widespread deltaic "packages" which stifled carbonate sedimentation over several hundred square-mi e (minimum) adjacent areas. (5) Thin shale or claystone breaks to featheredge argillaceous partings in limestone record flood-deposited prodeltaic increments. (6) Shale or claystone interbeds (more than 2 ft or .6 m thick) in limestone as well as shale- or claystone-sandstone units tens to a few hundreds of feet thick and laterally equivalent to largely limestone are predominantly the composite record of many floods and many times of small-scale mass movements. (7) Within the thicker terrigenous detritus units, individual flood or individual mass movement record is extremely difficult to define. (8) The lateral movement of life assemblages was slower than the influx of terrigenous clay in flood-generated plumes and organic communities were buried before they could vacate the area. (9) Te rigenous detritus sedimentation rates exceeded carbonate sedimentation rates. (10) The writer is unaware of a convincing modern analog supportive of all aspects of these conclusions. Possibly, the problem is that day-to-day tides reach most present-day shorelines whereas that may not have been so in the Paleozoic epieric sea setting.
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