ABSTRACT

A sedimentary rock is a product of its provenance and transportational mechanisms as well as its environment of deposition; it reflects the physical and chemical conditions under which the sediment was transported, deposited, and buried. The factors of size, energy and water depth combine to give some indication of the environments of deposition and these factors are represented directly or indirectly on the IES Log (in the Gulf Coast the "base line", corresponding to impervious beds is commonly shale, and peaks to the left correspond to previous beds, commonly sandstones). Therefore, it is necessary to graphically know three general types of environmental transitions and configurations shown on the log. Type "a" appears to be the most common transition in Gulf Coast stratigraphy and exemplifies a rapid change from one size fraction to another; this transition indicates an abrupt environmental change or a local unconformity. Type "b" is a gradual transition from sand to shale or vice-versa brought about by an interbedded sequence of sand and shale; locally this transition could be attributed to a gradual regressive or transgressive oscillation. Type "c" suggests a gradual size change such as graded bedding, and could appear either at the top of a transgressive phase or the base of a regressive phase; this type is least common and difficult to recognize. Hence nine different combinations, with minor modifications, are possible in a sand-shale sequence and these are used to interpret the possible environments of deposition of the beds.