"Y" City and it's rich display of geological formations.

Below is an in-depth discussion of the exposed sandstones, siltstones and shales that can be viewed right across Highway 71 from the "Y" City Mountain Inn. This outcrop was exposed by excavation crews during the construction of Highway 71 and provides a rich display of sedimentation layers for students of Geology, Geologists, and general public.  This outcropping as well as other rock structures provide spectacular scenery as well as recreation, as several of the rock structures are popular with rock climbers and for rappelling.

INTRODUCTION:

The Jackfork Group is a Pennsylvanian flysch succession of sandstones, siltstones and shales which outcrop in the Ouachita Mountains of western Arkansas and eastern Oklahoma.  The Ouachita Mountains form an east-west trending belt about 350 km long and 50 km wide. The Ouachitas are composed of Ordovician through Pennsylvanian strata. Ordovician, Silurian, Devonian and Lower Mississippian strata consist primarily of shales and cherts; these sediments represent slow deposition in a deep ocean basin. During the Mississippian Period the rate of deposition increased dramatically resulting in the thick succession of Mississippian and Pennsylvanian flysch deposits. During the Ouachita orogeny in late Pennsylvanian, strata of the Ouachitas were severely deformed.  As a result, compression -related folds and faults are common throughout the Ouachita Mountains

The Jackfork Group is included in the Morrowan Series of the Pennsylvanian System (Gordon and Stone, 1977).  The Jackfork conformably overlies the Stanley Group (Mississippian) and is conformably overlain by the Johns Valley Shale (Pennsylvanian). Jackfork strata range in thickness from 1500 to 1800 meters. The Jackfork units are exposed in two main areas in the Ouachita Mountains, the Athens Plateau south of the Broken Bow-Benton uplift and in the frontal Ouachitas north of the uplift.

The Jackfork succession of massive sandstones, debris flows, and turbidite sandstones and shales were deposited in a deep ocean basin.

Paleo-current data indicate a westward and axial direction of flow for the Jackfork units resulting in a westward building deep-sea fan (Clines 1960, Morris, 1974). The Pennsylvanian basin was bounded on the north by a continental slope and shelf; instability along this slope resulted in submarine slumps and slides. Consequently, debris flows and rubble bedding are common along the frontal Ouachitas where slump deposits and debris flows interfinger with turbidite deposits.

PREVIOUS INVESTIGATIONS

The Jackfork was named by Taff (1902) from exposures on Jackfork Mountain in southeastern Pittsburg County, Oklahoma.  Cline (1960) gives a complete history of nomenclature of the Jackfork Group.  Har1ton (1938.1959) divided the Jackfork Group of Oklahoma into five formations.  In ascending order the formations are: Wildhorse Mountain, Prairie Mountain, Markham Mill, Wesley and Game Refuge.  Morris (1964) proposed that the Jackfork Group in Arkansas be divided into the lower Irons Fork Mountain and upper Brushy Knob Formation in Arkansas because the siliceous shales used as stratigraphic markers in Oklahoma are absent in Arkansas. The type section

for both formations is on Forest Service Road 216, T1S, R28W, S13,14, 23, 24 (Morris, 1964). Previously there has been some uncertainty concerning the age of the Jackfork; however the discovery of early Morrowan ammonoids by Gordon and Stone (1969) has clarified the age assessment of the Jackfork Group, which is now included in the Morrowan Series of the Pennsylvanian System.

The Ouachita strata were interpreted as a flysch deposit by Cline (1960,1970) based on comparisons with the classic flysch facies of the Alps and Carpathians. The structure and tectonic history of the Ouachitas have been discussed by many workers including Viele (1973,1977,1979), Morris 1974b), Graham and others (1975), Wickham and others (1976), Thomas (1977), Morris and others (1975), Arbenz (1980), Stone and Ha1ey (1981), and Zimmerman and others (1982).  Deposition of the Jackfork strata have been discussed by Morris (1971a, 1971b, 1974a, 1977b), Thomson and LeBlanc (1975), Graham and others (1975), Locke (1979), and Stone and McFar1and (1981).  Petrographic studies include those by Bokman (1953), Go1dstein (1959), K1ein (1966), Morris (1977a), Proctor (1974), Morris and others (1979, and Stone (1981).

PROPOSED INVESTIGATION

A knowledge of the stratigraphy and sedimentology is necessary for a better understanding of the sedimentological and diogenetic history of the unit.  It is also necessary for a better understanding of the hydrocarbon potential the Jackfork may possess.  The Y-City section, located in Scott County, Arkansas in Sec. 22, T. IN., R. 29W., is the location of the study area (Figure I).  These rocks are exposed in a fresh outcrop and are easily accessible at the junction of Highways 71 and 270. The dominate facies in this section is the disturbed bedding comprising over 60% of the exposed section (Lyle, 1985).  The Y-City section is an excellent area to determine the slope versus axial fan origin of the units.