ABSTRACT

Mesozoic, Paleozoic, and Precambrian igneous rocks have been penetrated in the subsurface of northwestern Mississippi by the Pan American (Amoco) No. 1 Word, Sunflower County. This well encountered Late Cretaceous waterlain volcaniclastics, Late Jurassic trachytes, Middle to Late Ordovician basic intrusives, and Late Precambrian syenite.

The Cretaceous volcaniclastic sequence occurs 85 m below the top of the Eutaw Formation and is comparable to rock types penetrated in northwest Mississippi dated at 78.3 to 91.3 Ma. The Jurassic igneous rocks comprise an 805 m trachyte flow complex 270 m below the top of the Cotton Valley Formation and were dated at 174 ± 8 Ma. Macroscopically, they are gray-green to gray-black, very fine to finely crystalline, slightly porphyritic, amygdaloidal flow rocks which have been severely altered and locally intruded by Cretaceous-Age(?) dikes. The Ordovician igneous section is apparently restricted to basic igneous intrusives in the Precambrian syenite and has been dated at 468 ± 19 Ma. The deepest igneous sequence encountered by the No. 1 Word is a 306 m interval of white to greenish-gray, fine- to medium-grained Precambrian syenite dated at 785 ± 34 Ma.

The Precambrian section apparently formed during the rifting of North America and possibly represents the peripheral cratonic margin of Paleozoic North America. Tectonic uplift and erosion at the end of the Carboniferous Ouachita Orogeny removed the Paleozoic sedimentary section. The sole relict of this sequence is believed to be an 18 m conglomeratic quartzite of unknown age which lies between the Jurassic and the Precambrian.

The Jurassic flows emanated from volcanic features associated with the Mesozoic igneous complex of the Monroe-Sharkey uplift. Several eruptive phases are apparent from petrographic and seismic evidence. This section was exposed to subaerial erosion only briefly before burial by younger Jurassic sediments.

Between the Jurassic trachyte and the Cretaceous volcaniclastics are 878 m of sedimentary rocks of the Cretaceous Tuscaloosa Formation, undifferentiated Lower Cretaceous strata, and upper Jurassic Cotton Valley Formation. The Cretaceous volcaniclastics apparently reached the surface as tuffaceous material, were weathered, and redeposited to form the unit preserved today.

In conclusion, all of this igneous activity has been concentrated in northwest Mississippi at the confluence of the Precambrian cratonic rift margin, the Paleozoic - Mesozoic intracratonic Reelfoot Rift, and the Mesozoic Gulf of Mexico peripheral fault zone. Further, this tectonism has created important structural and stratigraphic traps that should be excellent exploration objectives.