Today the Trinity River flows generally west to east across the northern part of Jack County in the Fort Worth Basin, and it is paralleled a few miles to the south by the watershed that separates the Trinity from the somewhat larger Brazos drainage system in the adjacent counties of Young, Palo Pinto, and Parker. These fluvial features are geologically quite recent, having been developed in just the last few million years or less. It is not commonly known, but about 312,000,000 years ago, during the very late Mississippian and/or early Pennsylvanian Periods, a much larger and unrelated river system flowed anonymously through the center of Jack County.
During most of the late Paleozoic Era, North Texas was covered by shallow epicontinental seas that only occasionally retreated far enough to allow the most recent sediments to be exposed to subaerial erosion. Two such erosional episodes occurred during the late Mississippian and/or early Pennsylvanian Periods. In most areas of Jack County, rocks of the Morrowan and Atokan Epochs (earliest Pennsylvanian) directly overlie rocks of the Chesterian Epoch (latest Mississippian Period) Forestburg Formation unconformably. Indeed, a Mississippian-Pennsylvanian hiatus has long been known and mapped across most of the mid-North American continent, so it is no surprise to find it in the Fort Worth Basin. Together, the Marble Falls and the Forestburg occupy a vertical interval of very approximately half a thousand feet throughout most of Jack County. The two formations thicken and thin in an inverse manner with each other so as to indicate that the Marble Falls was deposited on a near shore terrain and preferentially filled in the paleotopographic lows in the same way a cast would fill a mold. Consequently a detailed isopach of the Forestburg in Jack County (See Figure 1 below.) shows a dendritic pattern that has the classic and unmistakeable symmetry of a subaerial drainage system.
Today buried beneath more than 5000 feet (1500 meters) of early- to late Pennsylvanian sediments, the main channel is oriented generally NNW-SSE and flowed through the county in a generally southerly direction as it passed barely a mile east of the present townsite of Jacksboro. The here-named Jacksboro River was a very real and well developed fluvial pattern that meandered across the low-standing paleoplain of mid-North America during the widespread Mississippian-Pennsylvanian erosional hiatus. The headwaters of the river presumably originated in the rising Wichita Mountains in southern Oklahoma a few dozen miles or so to the north.
The age of the Jacksboro River can be narrowed down to an interval of just a few million years. While the latest-Mississippian Chesterian Epoch endured for a rather lengthy 15 million years (333-318 million years before present), the succeeding early-most Pennsylvanian Morrowan Epoch lasted a relatively brief six million years (318-312 mybp), and the immediately-following Atokan Age persisted for only four million years (312-308 mybp). While in most of the Fort Worth Basin the Atokan Marble Falls rests directly on the Chesterian Series, there is an area of several hundred square miles in northern Jack and southern Clay Counties where the former strata are clearly found to be unconformably overlying isolated beds of Morrow Shale that are as much as 120 feet (36 meters) thick. (For a more detailed discussion of the Morrow, refer to “The Marble Falls, the Morrow, and the Forestburg” page elsewhere on this website.) It is no coincidence that the deeper parts of the Jacksboro River valley are where the only remnants of the Morrow Shale are preserved. This strongly suggests that the Chester/Morrow unconformity in the Fort Worth Basin is truly correlative with the widespread erosional surface that covers almost all of mid-North America, and that it was this erosional event that is most responsible for the paleotopograpy that we find beneath the Atokan Marble Falls strata. A subsequent erosional interval at the unconformable Morrow/Atoka contact undoubtedly modified that ancient terraine, but the deepest parts of the Jacksboro River valley had already been carved before the Morrowan seas moved back into the area.
Correlative Morrowan strata in the Marietta, Ardmore, and Anadarko Basins to the north and east exhibit no indications of faulting, folding, or tectonism during the time they were being deposited, and it appears that the remnants of those fine grained Morrowan rocks in the northern Fort Worth Basin were originally blanket beds laid down in a shallow sea on the southwestern shelf of the northwest-southeast-trending Southern Oklahoma Aulacogen. At first, the Jacksboro River may have been an estuary of that shallow sea. The very approximate thickness of what are generally assumed to be Atokan-aged sediments just above the Chester and Morrow strata is roughly 1000 to 1500 feet (305 to 460 meters) in this area. While we can not know the exact rate of sedimentation in Jack County at that time, we might guess that it required a substantial fraction of the time allotted to that four-million-year span to account for the Atokan rocks that have been preserved there. My best guess for the age of the Jacksboro River is about (313-310 mybp). To my knowledge, no other Morrowan rocks are known to be preserved anywhere in the Fort Worth Basin.
Elsewhere, Siever (1951) documented the exact same mid-Carboniferous unconformity with closely analogous drainage patterns in the southern Illinois Basin. Subsequently, Droste and Keller (1989) were even more meticulous in describing this comparable feature in southwestern Indiana and went so far as to give names to nine different pre-Pennsylvanian river systems that developed on the broad, late Mississippian peneplane.
We can follow the deeply incised main channel of the Jacksboro River at least as far down the paleoslope as the southeast corner of Jack County where it debouched into Palo Pinto and Parker Counties. No other area of preserved Chesterian sediments exhibits the magnitude of erosional paleotopography that the drill bit has found in Jack County. The buried Forestburg limestone hills that flank the Jacksboro River valley rise at least 500 feet (150 meters) above the old river bed on both the west and east. Moreover, the narrow main channel positioned between the twored “zero”lines on the isopach map above cut down completely through the Forestburg and some distance into the underlying Barnett Shale, perhaps as much as 100 feet (30 meters) in a few places. Add to that the erosional absence of some uncertain thickness, possibly 120 feet (36 meters) or more, of Morrow Shale that otherwise would have been preserved in Jack and surrounding counties, and the Jacksboro River valley could have once been at least as deep as 650 or 700 feet (200 to 215 meters). That would have made it a considerably more formidable stream than either the present-day Trinity or Brazos Rivers.
A preliminary and much less detailed study of the Miss/Penn unconformity in adjacent Montague County to the northeast shows no evidence of tributaries feeding into the Jacksboro River from that direction. A structure map on that unconformity will show that it dips very gently from the quad-corner area of Jack, Wise, Clay, and Montague Counties in a northeasterly direction at an angle of about one-half degree until it is abruptly truncated against the southwestern fault boundary of the Muenster Arch.
