KARST HYDROGEOLOGIC DEVELOPMENT IN THE BOONE FORMATION (LOWER CARBONIFEROUS) OF THE SOUTHERN OZARKS

The Boone Formation of northern Arkansas and age-equivalent units in contiguous states is a well-developed karst aquifer, although the presence of significant chert in the formation mantles and masks traditional karst landforms at land surface. Shallow- and deep-water carbonates of the Boone have been uplifted and systematically jointed by far-field effects from early stages of the Ouachita orogeny, and exposed to a sequence of subareal dissolution from both hypogenetic and epigenetic groundwater sources that facilitated the karstification of the carbonate rocks. Overprinting of karst development is widely observed throughout the area.

Chert and limestone in the Boone occur in couplets that reflect what is thought to be caused by variable pulses of volcanic ash from sources occurring to the south of the present Ouachita Mountains. In some areas of northern Arkansas, as many as 60 couplets are present in the middle to lower Boone, many with continuous distribution of chert layers that range from less than several centimeters to many tens of centimeters thick. These extend across regions that can be greater than tens of kilometers². Limestone production was continuous, but is only lithologically dominant during periods of volcanic quiescence. Many, although not all of the intervening limestone layers of the couplets have been karstified, producing stacked flow zones that are confined by the chert, and encountered during differing groundwater levels in response to varying recharge inputs. Systematic jointing and reactivation of preexisting basement faults define groundwater basin boundaries, which are typically low flow. Slight variations in tilting from the pulses of uplift have resulted in dispersive groundwater flow patterns from recharge areas to springs, flow patterns that have been well-documented by numerous karst researchers during multiple dye-tracing tests in the area for a wide range of hydrologic-flow conditions. Although hydrogeologic development is atypical for the Boone when compared to many other karst aquifers, observed karst processes across a wide range of scales are rationally explained and consistent within existing geologic understanding.