Abstract

Cyclothems of Late Mississippian age in southeastern Scotland were quantitatively analyzed to determine the evolution of a delta. Each cycle was divided into transgressive or destructional (coded "A"), progradational (coded "B + C"), and aggradational (coded "DE" or "E") phases.

In the early period of deltaic development (lower part of the section), the modal cycle pattern is ABCDE, where the "D" unit is interpreted as a distributary sandstone, and the "E" unit consists of rooty beds and/or coal. In the intermediate period of delta development, the modal cycle pattern is ABCE (i.e., no distributary sandstone between the progradational and aggradational phases). In the later period of delta development (top of the section), multiple transgressive and progradational phases (e.g., ABCABCDE) are found, some showing bioturbated cross-bedded sandstone ("X" units; e.g., ABCXABCE). The later portion of the delta shows relatively thin aggradational vs transgressive and progradational phases within each cycle. This is in contrast to the earlier portion of the delta which contains very thick aggradational phases vs transgressive and progradational phases within each cycle. When salinities of the transgressive phases throughout the section are considered, it becomes apparent that the evolution of the delta was as follows:

After a period of redbed fluvial and alluvial fan deposition (Late Devonian - Early Mississippian, Upper Old Red Sandstone), a high constructive elongate delta developed into a shallow lagoon (Mississippian, Calciferous Sandstone Measures). The best recent model for this delta is the Guadalupe Delta, San Antonio Bay, Texas. The Scottish delta gradually evolved into a wave-dominated destructive delta by later Mississippian time (Lower Limestone Group). The lagoon into which the delta first prograded (Lower Visean time) remained shallow and mainly brackish water (oil shale, shale, cementstones; occasional marine bed), and the units are traceable, generally, from 2 to 6 miles. In the upper portion of the section (Upper Visean time), marine units developed that are traceable as far west as the Glasgow area.