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The Gallup Sandstone of northwestern New Mexico is a northeastward- prograding clastic wedge of late Turonian to earliest Coniacian (Late Cretaceous) age that pinches out about in the middle of the San Juan basin. Paleoenvironmental and sequence stratigraphic studies indicate that the Gallup is dominated by strand plain successions (tongues) that prograded across a gently dipping ramp during repeated episodes of relative sea level fall. These episodes were superimposed on a long-term (about 1.2 m.y.) phase of relative sea level rise that controlled the overall forestepping and aggradational stacking pattern of the tongues. The total stratigraphic rise of all six Gallup tongues is about 120 m.

The Gallup is divided into chronostratigraphically significant packages that are bounded by mappable surfaces of erosion and their downdip conformities. Outcrop studies present incontrovertible evidence, for at least three of the Gallup tongues, that two concurrent erosional surfaces formed during sea level falls in this ramp setting. The lower erosion surface forms a sharp base of the shoreface and is referred to as a regressive surface of marine erosion. This erosion surface generally correlates with conformities both updip and downdip. The upper surface commonly juxtaposes estuarine and fluvial sandstone on truncated shoreface successions and is referred to as a regressive surface of subaerial erosion. We consider this upper surface to be the sequence boundary. The strata between these two erosion surfaces belong to the falling stage systems tract. The sequence boundary climbs stratigraphic section (relative to the base of overlying shale) from landward to seaward and becomes a conformity near the position of the lowstand shoreline.

There is no mappable, erosional sequence boundary seaward of the shoreface sandstones. Transgressions (floodings) were associated with deposition of thick coastal plain strata of the Crevasse Canyon Formation, partial truncation of these strata by marine ravinement, and formation of a fining-upward succession of offshore marine deposits. The ravinement surface climbs stratigraphic section from seaward to landward.

The Gallup sequence architecture indicates that periods of relative sea level fall were associated with progradation of shoreface sandstones, aggradation of offshore mudstones, and erosion on the coastal plain forming a sequence boundary. Conversely, during (most of) the phase of relative sea level rise, there was deposition on the coastal plain, erosion on the ravinement surface (transgression), and some deposition offshore. We believe that this “reciprocal” style of sedimentation during sea level fall and rise is characteristic of many Western Interior Cretaceous sequences, even those that form during very short intervals of time.

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