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Abstract

Middle Ordovician graplolite-rich shales exposed in the southern Appalachians provide an opportunity to apply graphic correlation to what appear to be inadequate data. The fact that the resulting composite standard (CS) is used successfully to address significant geologic problems demonstrates the effectiveness and versatility of the technique. The shales are so strongly diachronous that no single stratigraphic section spans the entire biostratigraphic interval represenled by the shales. The Standard Reference Section (SRS) included only a part of this interval. Therefore, many partially overlapping sections had to be used in the construction of the CS with each section extending the CS upwards or downwards. The completed CS contains the data from 22 stratigraphic sections, is composed of 89 composite standard units (CSUs), and ranges from the lower G. teretiusculus Zone to the C. bicornis Zone.

Once we established the CS, we used it as a basis for correlating the basal shale contact. This contact records the subsidence and migration of the foreland basin in which the shales were deposited, and its diachroneity can be expressed in terms of CSUs. With the biostratigraphic correlation of radiometric dates into the CS, the duration of the CSUs could be calibraled in terms of years, and so, too, could the age differences between sections of the diachronous basal shale contact. Because this contact recorded the migration of the basin axis, we used its age difference and the palinspastically restored distance between sections to calculate migration rates. Calculated rates indicate that the basin migrated 50 km cratonward at an average of 13 mm/yr. We were also able to demonstrate that the rate decreased from 40 mm/yr to 9 mm/yr before the migration completely cease4. The foreland basin migration and its subsequent deceleration and halt were produced by arc-continent convergence and collision. Our calculated rate of 40 mm/yr is comparable to modern rates of convergence.

We also calculated sediment accumulation rates for selected sections. To do so, we used the slope of the line of correlation (LOC) to determine the number of CSUs and their duration in years corresponding to the stratigraphic thickness of graptolite shales in the section compared to the CS. The rates of 1.8 to 2.8 cm/1000 yrs calculated for intervals of 1.3 to 2.8 Ma are within the ranges of those reported in other studies of pelagic environments.

Our rates of basin migration and sediment accumulation are virtually identical to rates determined from a variety of other methods, demonstrating the validity of a CS constructed from what appear to be inadequate biostratigraphic data.

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