Laterally Discontinuous Porous Zones in Subsurface Mississippian Monteagle Limestone (Oolitic), Northeastern Tennessee--How Were They Formed?

Richard E. Bergenback

Examination of over 500 logs composed of gamma-ray, bulk-density, and porosity graphs that penetrated the Mississippian platform carbonate unit known as the Monteagle Limestone (oolitic) in the subsurface of Morgan County, Tennessee (Twin Bridges and Pilot Mountain Quadrangles), enabled recognition of laterally discontinuous porous zones that formed at eight different stratigraphic levels within the Monteagle.

Seven panel diagrams per quadrangle (a total of 14 panels) were prepared to ascertain the three-dimensional geometry, facies relationships, and distribution of porous zones of subsurface stratigraphic units in the Mississippian System. These panels may be considered as substitutes for seismic traverses.

It is well known that oolitic tidal-bar deposits form more or less perpendicular to carbonate platform margins. Therefore, it is suggested that these laterally discontinuous porous zones formed by subaerial leaching along the tops of tidal bars during times of lowered sea level.

Certain of the porous zones within the Monteagle may represent Peter Vail's worldwide eustatic changes, but in the absence of paleontologic control, it is impossible to determine which of the porous zones are associated with eustatic changes. Therefore, the next best interpretation proposes that localized transgressions and regressions account for porosity development in these platform margin oolitic mounds or bars.

Sequence-bounding unconformities have been documented in the subsurface of Fentress, Morgan, and Scott Counties in northeastern Tennessee. The lower unconformity shows a karst surface developed on Ordovician carbonates, which is overlain by a Devonian Chattanooga Shale drape. The upper unconformity displays scouring in the uppermost Mississippian Pennington Formation with lowermost Pennsylvanian Fentress Formation infilling erosional lows of the Pennington.

Further, Paleozoic depositional basins are present both north and south of this area of repeated tectonic uplift.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.