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The Northwest Hartburg field, located in southern Newton County, Texas, is in a structural belt which forms part of the Gulf Coast Frio (Oligocene-Miocene) producing trend and for which the name Hartburg flexure is suggested.
The various structural and depositional features (folds, faults, truncation, channels, etc.) observed in this trend are particularly well exhibited in the Northwest Hartburg field area and indicate a structural development which, reaching a climax during earlier Frio time, can be divided into four stages.
During stage 1, continental shelf conditions prevailed throughout the area and a sedimentary series, consisting essentially of shale in its lower and a sandy section in its upper part, was deposited on the slowly subsiding shelf. The sand section is subdivided, in ascending order, into the Nodosaria sand, and lower and upper Hartburg sands, the latter two names replacing the more commonly used but misleading names "lower and upper Hackberry sands."
Toward the end of upper Hartburg sand sedimentation, the area experienced a large scale and semi-regional structural disturbance in the form of a breakdown of a segment of the Gulf Coast geosynclinal floor. This event tilted the continental shelf, producing the Hartburg flexure (scarp) and an embayment which was later filled by the Hackberry shale.
The tilting of the continental shelf area induced gravity flow in the sedimentary beds which was essentially a downfolding process. The resulting synclinal folds formed deep troughs and trenches turning the scarp into one of considerable relief that resembled the topography of the present continental slope of the northwestern Gulf of Mexico. Most of the troughs show effects of local submarine erosion in the form of channels and scour scars, some of which extended also across the anticlinal fold adjoining the trough. Besides tilting the continental shelf, the regional downwarp also lowered the base level of erosion and depositions across the shelf area. In adjustment to the new profile of equilibrium the anticlinal folds were truncated as evidenced by the presence of a local angular uncon ormity between the Hackberry shale and underlying formations.
After gravity flow subsided and the new base level of erosion was attained, turbidity currents and sedimentary flows filled the troughs and channels with "channel-fill" deposits.
On completion of the peneplaning action, the deposition of the Hackberry shale commenced restoring again to the area more normal depositional conditions. Mild folding and faulting continued or were rejuvenated during this and later depositional cycles on some of the local folds in the Hartburg trend.
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