Page 199 - Carbonate Facies in Geologic History

186 Pennsylvanian-Lower Permian Shelf Margin Facies

Fisher Jones
MIDLAND
BASIN

----~~-,~~~--~~--~--------~~
DEEP WATER e ~
~ *
~ 0
Q. SHELF
~~
~o~~'---~~~R~u-nn-e~ls--'-~------~

o 50km ___ ~
! .... --------....j!
Fig. VI-14. Large carbonate banks of Late Pennsylvanian age in Midland basin off the west
edge of cyclic Eastern shelf, subsurface of north central Texas. Counties are named. Offshore
banks from north to south are termed: Claytonville, Rowan and Hope NW, Rowan and
Hope, Esteban, Stone and Lake Trammel, Nena Lucia, Jameson, I.A.B., and Millican

The greatest such bank is the "Horseshoe Atoll", a complex of buildups rising
from a Middle Pennsylvanian platform encompassing about eight counties in
north central Texas and lying in the northern part of the Midland basin. Fig-
ures VI-15 and VI-16 are isopach and stratigraphic layer maps of this field. The
initial large, equidimensional platform (40 km in diameter) accreted on opposite
sides of a gentle arch trending north-south in the basin. About 100 m of marginal
relief developed chiefly through accumulation of detrital shallow water carbonate
building up above dark limestone and shale. During later Pennsylvanian time
continued subsidence in the basin was matched by continued organic carbonate
accumulation around all margins of the platform except the north. This arcuate
buildup plus the added Wolfcampian accumulation on the southwestern side of
the Horseshoe totals 900 m. The equivalent basinal strata total only 150 m. The
750 m on the western side and about half that in the Pennsylvanian on the eastern
side may represent the approximate total relief of the buildups; part of this could
be erosional since major disconformities are discerned at the base and on top of
Cisco (Virgil ian) strata. Numerous shale "breaks" can be traced across the
buildup showing that the thick carbonates are basically detrital masses some of
which had only limited relief during their formation.
Petrographic study across the eastern and Pennsylvanian prong of the Horse-
shoe found algal plate mounds and detrital wackestones with abundant fusulin-
ids, oolitic grainstones, and encrinites forming flank beds. Sedimentary bodies
within the major buildups have not been distinguished but configuration of iso-
lated areas would indicate elongate mounds a few km long and 100 m high, more
or less parallel to the shelf margin trend of the platform. Channels have been