The Bisti field of the San Juan basin is a classic example of a bar type stratigraphic trap in the Gallup Sandstone of Late Cretaceous age. The various sedimentary facies are defined and described together with petrographic criteria for recognizing each facies. The sedimentary facies and their characteristics are as follows.

1. Main Gallup Sandstone--non-productive widespread regressive marine sheet sandstone consisting of

a. Offshore sand facies--very fine-grained sandstone with abundant primary dolomite grains and little or no glauconite. Southwest of Bisti field this facies is overlain by the Beach sand facies.

b. Beach sand facies--medium-grained sandstone with low to moderate content of primary dolomite grains and little or no glauconite.

2. Low SP facies--poorly sorted sandy shale and shaly sand with low to moderate content of primary dolomite grains and little or no glauconite. This facies underlies each of the productive bar sands and separates them from the underlying Offshore sand facies.

3. Bar sand facies--medium-grained sandstone with little or no primary dolomite grains and very abundant glauconite. The three individual bars are here designated as the Marye, Huerfano, and Carson Members of the Gallup Formation. On their seaward (northeast) margin the bar sands grade into the Fore-Bar facies; on their landward (southwest) margin they grade into the Back-Bar facies.

4. Fore-Bar facies--marine shale characterized by an open marine fauna of planktonic Foraminifera, collophane, and Inoceramus.

5. Back-Bar facies--marine shale, but of a restricted marine environment lacking the open marine fauna; pyrite-filled benthonic Foraminifera are common.

6. "Upper" Mancos Shale--shale of open marine environment overlying the entire Gallup Formation.

The stratigraphic nature of the Bisti trap is further demonstrated by the absence of closure on structural maps of horizons above and below the bar sands.

In the bar sands clay and microcrystalline calcite are the two types of matrix. Megacrystalline calcite and secondary quartz are the two types of cement.

Depositional history of the Bisti stratigraphic trap began with regressive deposition of the Main Gallup Sandstone. This regression was interrupted by a pulse of subsidence, and possibly a minor disconformity, after which the sandy mud of the Low SP interval was deposited. Wave action winnowed the mud from this sediment and concentrated the sand as a series of bars. A restricted marine environment existed on the landward side of the bar complex and an open marine environment on the seaward side. A basin-wide pulse of subsidence resulted in an abrupt marine transgression which buried the bar complex beneath the "Upper" Mancos Shale.

End_Page 193------------------------------