Geological Survey Professional Paper 1044—C The Waters of Hot Springs National Park, Arkansas—Their Nature and Origin

The rocks cropping out in the vicinity of the hot springs are sedimentary rocks, although intruded igneous rocks are exposed in the region (Purdue and Miser, 1923). The sedimentary rocks are relatively old (Paleozoic) and consist of shale, chert, novaculite, and sandstone. The names of the geologic formations, their geologic ages, and positions in the geologic column are given in table 1.

TABLE 1.—Generalized section of sedimentary rocks in the vicinity of the hot springs
(Modified from Purdue and Miser, 1923]

Though no significant igneous rocks are exposed in the immediate vicinity of the hot springs, their nearby occurrence has been frequently cited in literature as possible sources for the heat of the springs. The igneous rocks were intruded into the sedimentary rocks during the early Late Cretaceous time (about 90 million years ago). The larger igneous intrusions in the hot-springs region are exposed in two small areas, about 6 miles southeast of the hot springs. Elsewhere in the region, igneous intrusions occur as very small dikes and sills. The sedimentary rocks in the vicinity of the hot springs were originally laid down on a sea bottom of nearly horizontal beds. At present the beds are generally steeply inclined, because of tremendous and complex mountain-building forces in late Paleozoic time. The rocks have been subjected to at least three episodes of structural deformations—two episodes of compression from the south, producing imbricating thrust faults and, third, forces from the north which produced overturning and folding of beds and fault planes and further faulting. The geologic map (pl. 1) shows the edges of the inclined strata where they intersect the land surface. When the formations are crossed from northwest to southeast, they are seen in cross section (pl. 1) to lie in a series of very complexly folded anticlines and synclines, with some associated thrust faults.

The hot springs emerge from the plunging crestline of a large overturned anticline along the southern margins of the Ouachita anticlinorium in the Zigzag Mountains. The Zigzag Mountains basically owe their presence to the resistant exposures of the Arkansas Novaculite. The zigzag pattern of the strata is mostly due to tightly compressed folds which plunge southwestward into the Mazarn Basin. The Mazarn Basin is a structural and topographic basin lying south of the Zigzag Mountains. The Stanley Shale, a formation much less resistant to erosion than the Arkansas Novaculite, crops out at the surface of the Mazarn Basin. The structural setting is illustrated by figure 6 in Purdue and Miser (1923).

The formations, composed predominantly of shale, include the Womble, Missouri Mountain, Polk Creek, and Stanley Shales. The shales have low permeability, but, locally, limestones in the Womble yield water to springs. Shales generally impede ground-water movement, except where open joints and fractures are present. Wells in shales generally yield meager quantities of water; recharge to shales is also small.

The Bigfork Chert typically is highly permeable, exhibiting intergranular and fracture permeability. The Bigfork Chert is composed of silt-sized, generally poorly cemented siliceous particles in thin beds 1/2 to 4 in (13 to 130 mm) thick, which have been weathered, leaving a friable material, interbedded with layers of dense chert 4 to 12 in (130 to 450 mm) thick. The dense chert beds were rendered permeable by fracturing, which accompanied the intense folding of the beds, whereas the decalcified silt-sized material has significant intergranular permeability near the ground surface.

Wells that yield the largest quantities of water in the region tap the Bigfork Chert. At Belvedere Country Club, northeast of Hot Springs, the Bigfork, tested by Albin (1965), was found to have a transmissivity of 2.67 x 10³ ft/d (9.42 x 10^-3 m/s). Many of the springs in the area issue from the Bigfork Chert, and many of the cold-spring emergences are controlled by contact of the Bigfork with adjacent, less permeable formations. This association of cold springs with the Bigfork Chert was noted by Purdue and Miser (1923).

The Arkansas Novaculite is composed of three divisions—an upper and a lower division of novaculite, and a middle division of chert. Locally, the upper part of the formation is composed of silt-sized siliceous particles and possesses intergranular permeability. The lower division is generally massive and dense, but is very closely fractured. The middle division is composed mostly of black shale and thin chert beds. The Arkansas Novaculite is not as permeable as the Bigfork Chert, but is locally intensely jointed. Some cold springs issue from, and many water wells tap, the Arkansas Novaculite.

The Hot Springs Sandstone Member of the Stanley Shale is a massive, quartzitic sandstone. Fairly large joints and fractures, as in the novaculite, create some highly permeable conditions, such as at the hot springs.

The hot springs emerge from the Hot Springs Sandstone Member near the axis, on the northwest limb, of a southwestward-plunging anticline. The springs emerge between the traces of two thrust faults that are parallel to the axis of the anticline (pl. 1). The locations of the hot springs, shown in figure 2, generally lie along several northeast-trending lines. According to Bryan (1924), these lineaments were inferred to be the traces of fissures by R. R. Stevens, who first noted their alinement in 1890. Jointing is common in the few exposures of the Hot Springs Sandstone Member in the hot-springs-discharge area. Thus, the hot springs are associated with thrust faults, and with normal faults and joints, on the plunging crestline of the anticline. Upward movement of the hot waters from depth is probably along the permeable fault zones. These fault zones probably carry water to near the surface, where the water follows permeable joints to the spring outlets.

Geologic sections in plate 1 show the geologic structure in the vicinity of the hot springs.