About This Item
Share This Item
Mesozoic Igneous Activity in the Northern Gulf Coastal Plain: Abstract
Igneous rocks of Mesozoic age occur at widely scattered localities in the central part of the United States. Some of the surface exposures, such as the intrusive stocks at Little Rock and Magnet Cove, Arkansas; the peridotite pipes near Murfreesboro, Arkansas; and the lamprophyric dikes of southern Illinois have been known for many years. Deep drilling in the Northern Gulf Coastal Plain has shown the presence of many additional igneous occurrences, particularly in the Monroe uplift and Jackson dome. More than 100 wells have encountered primary igneous rocks, and many others have penetrated pyroclastic debris.
The diabase province includes at least 14 known occurrences in northeastern Texas, southwestern Arkansas, and western Mississippi. None of these rocks are exposed at the surface. They occur as thick dikes and sills in the Eagle Mills or at the base of the Cretaceous where the Eagle Mills has not been encountered. The age is pre-Upper Jurassic and may be the same as the similar Triassic diabases of the eastern United States. They represent relatively rapid emplacement of a basaltic magma into zones of tensional movement.
The alkaline province includes a greater variety of igneous rocks ranging from the truly alkaline rocks (nepheline syenite, tinguaite, phonolite, etc.) to normal syenites, lamprophyres, peridotites, tuffs, and volcanic breccias (diatremes). The age of emplacement ranges from lower Woodbine to lower Taylor, based on the intrusive relationships and the associated tuffs. The activity thus occurred during a relatively long geological span, during which there was sufficient magmatic differentiation to produce a variety of rock types.
There appears to be little relationship between the presence of igneous rocks and the general absence of oil or gas, except in a few places where larger dikes and stocks have caused rather severe metamorphism. The presence of relatively thin dikes and sills, such as those encountered in the Big Creek-Delhi field of northeastern Louisiana or the Omaha pool of Gallatin County, Illinois, seems to have had little effect on the producing capabilities of otherwise favorable reservoir rocks. As a general rule, the width of the zone adversely affected by an intrusion is no greater than the thickness of the intrusive itself.
Acknowledgments and Associated Footnotes
1 Esso Production Research, Houston
Copyright © 2006 by the Tulsa Geological Society