Structural mapping of the base of the Beech Creek (Barlow) Limestone across Louden oil field, Fayette County, Illinois, reveals a northwest-southeast trending saddle that is more than 1.5 km wide and 6 km long, and is perpendicular to the major axis of the Louden anticline. This depression coincides with the abrupt appearance of a thick, fine-grained, argillaceous limestone (so-called "false Barlow") subjacent to a regionally normal thickness of coarse-grained, bioclastic Beech Creek Limestone. Sandstone beds in the Cypress Sandstone, which generally underlie the Beech Creek, are thin or absent beneath this area of false Barlow.

This feature is believed to be a major tidal channel that breached deposits of shallow marine or eolian sands that had accumulated along the crest of the anticline. The trend of the channel, perpendicular to the anticlinal axis, and the restriction of the channel to the crestal area only, with no apparent extension off-structure, strongly suggest that the Louden anticline was topographically high during Cypress deposition. The channel was filled during latest Cypress deposition by marine shales and fine-grained limestone (false Barlow). During the main phase of sand deposition, the channel profoundly influenced local sandstone depositional patterns; two thick, offshore sand bars or barrier islands accumulated near its southeastern terminus along the flank of the anticline. These flank ng sand bodies pinch out updip against lagoonal shales and are true stratigraphic traps that have since produced several million barrels of petroleum.

The recognition of large marine bar sand bodies in the Cypress Sandstone opens new prospects for oil exploration in the Illinois basin. Henceforth, Cypress sandstones should not be viewed as massive blanket sands or overlapping fluvial channel sands, but rather as complex sequences of shallow marine sandstone. Favorable areas to explore for stratigraphic traps in the Cypress include the flanks of major anticlines, areas of thick false Barlow, and near linear gaps in the areal distribution of Cypress production.

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