Upper Cretaceous sandstone of the lower Tuscaloosa Formation in southwestern Mississippi is part of a fluvial-deltaic depositional system. At the Mallalieu field, lower Tuscaloosa sandstone is of two types: (1) channel-fill sandstone--thin, lenticular bodies which have irregular distribution across the field; and (2) point-bar sandstone--thick, more continuous bodies which have a ridge-and-swale pattern of sandstone distribution and which laterally are terminated

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abruptly by narrow, broadly arcuate, shale-filled channels. These two fluvial interpretations are supported by mineralogy, textural gradation, internal structures, and sand-body geometry.

The average composition of lower Tuscaloosa sandstone is quartz, 60 percent; matrix, 32 percent; calcite cement, 4 percent; feldspar, 1 percent; muscovite, 1.5 percent; and other minerals, 1.5 percent. Average mean grain size of quartz is 0.24 mm (fine grained); mean grain size decreases upward within individual sandstone beds.

Four distinct sandstone zones produce oil at Mallalieu. The lower two zones are characterized by more extensive, point-bar sandstone whereas the upper two zones are narrow, channel-fill sandstone. This vertical sequence suggests an upward gradation from fluvial meander-belt deposition, through deltaic distributary deposition, to inner neritic deposition of the overlying marine shale--an overall transgressive sequence. The change from meandering below to braided above probably resulted from a change in stream gradient by basin subsidence.

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