Depositional Environments and Diagenesis of the Montoya Dolomite, Sylvan Shale and Fusselman Dolomite, West-Central Borden County, Texas

Trentham, Robert and Teri McGuigan, Department of Geology, The University of Texas of the Permian Basin, Odessa Texas

Four wells in the Luck Pot (Fusselman) Field in west central Borden County, Texas have been cored in the Fusselman (Upper Ordovician(?) or Lower Silurian), Sylvan Shale  (Upper Ordovician) and Montoya (Upper Ordovician) Formations. The Luck Pot Field is one of a number of stratigraphic traps that produce along the eroded edge of the Fusselman in the eastern Midland Basin.

The Montoya Formation is composed of shallow subtidal, bioturbated dolowackestones and, in one well, of skeletal rich dolopackstones with leach moldic porosity. Chert nodules with ¼”(+/-) thick tripolitic weathering rinds are also present. These chert nodules have previously been described in the Dollarhide Field located 90 miles to the west in Andrews County, Texas, where it has been interpreted they have been eroded from pre-existing sections, carried across the carbonate mud flats and deposited in the intertidal environment. At Luck Pot the chert nodules were possibly deposited in the bioturbated, peloidal, skeletal, subtidal dolowackestones to dolopackstones during storm events. Also present are higher energy skeletal dolopackstones which are devoid of the chert nodules. The fact that these nodules are found 90 miles apart indicates that this phenomena is not restricted to the Dollarhide area. The source of the nodules has not been determined. The karst overprint on the Montoya includes vertical to subvertical breccia pipes. Some of the chert nodules were broken across the rinds during karstification supporting the model that the nodules were weathered prior to deposition in the interval.

The Montoya – Sylvan contact is unconformable. In addition to the vertical karst in the upper part of the Montoya, there is a lag deposit on the unconformity which is composed of rounded pebbles, some which are pyrite replaced. The Sylvan is a thick bedded green shale with blocky to platy partings. The upper portion of the Sylvan is thinner bedded with increasing carbonate content, some bioturbation, and lacking the platy or blocky partings seen in the lower Sylvan.

The contact between the Sylvan and the Fusselman can best be described as transitional. There is no evidence of weathering of the Sylvan, nor a lag deposit or soil profile. The base of the Fusselman is designated as the point where the content of shale drops rapidly. The contact between the Sylvan and Fusselman lacks evidence of a significant hiatus, supporting previous biostratigraphic work which placing the Lower Fusselman in the Upper Ordovician.

 The Lower Fusselman is composed of a series of shallowing upward wackestone to packstone cycles with leach moldic porosity in the higher energy skeletal packstones and interparticle porosity in the finer grained peloid wackestone. The top of the Fusselman is not seen in any of the cores.