Geology of the Arkansas bauxite region / Геология Арканзасского бокситоносного региона

The Arkansas bauxite region lies near the center of the State in Pulaski and Saline Counties. It is irregular in outline, covering about 275 square miles and is elongate in a northeasterly direction. Bauxite deposits in the region can be grouped into two principal districts, one in Pulaski County about 5 miles south of Little Rock, and the other, more important commercially, about 25 miles southwest in Saline County.
The bauxite deposits lie entirely within the Gulf Coastal Plain province a short distance southeast of its boundary with the Interior Highlands province. The surface rocks in this part of the Gulf Coastal Plain are early Tertiary sand and clay. The topography consists of gently rolling hills that are less than 200 feet above the valleys. The maximum relief is about 340 feet. The lowest point, on Fourche Creek, is about 240 feet above sea level, and the highest altitude, on Alexander Mountain 15 miles to the southwest, is 580 feet.
The rocks underlying the Interior Highlands province at the northeast edge of the bauxite region are folded sandstone and shale beds of Paleozoic age. North and northeast of Little Rock the topography consists of flat to rounded ridges and broad to narrow valleys. This area merges southwestward into a region consisting of prominent straight ridges and narrow valleys, generally trending northwestward. The Paleozoic rocks generally are increasingly old and more tightly folded to the southwest. The line of demarcation between the Interior Highlands and Gulf Coastal Plain physiographic provinces is the contact between the hard rocks of Paleozoic age and the soft beds of Tertiary age to the southeast.
The Coastal Plain includes marine and nonmarine strata of Paleocene and Eocene age, gravel of late Tertiary and Quaternary age, and terrace deposits and alluvium of Quaternary age. The Tertiary sediments rest unconformably on rocks of Paleozoic age and on intrusive masses of nepheline syenite of probable Late Cretaceous age. These strata lap over and around the resistant masses, particularly the nepheline syenite, parts of which remained exposed as hills throughout much of their history. The Tertiary strata generally strike northeastward and dip gently southeastward, although this has been modified locally by compaction. <...>