OUTCROP AND SUBSURFACE SEQUENCE STRATIGRAPHY AND PALEOGEOGRAPHY OF THE UPPER CRETACEOUS GALLUP SYSTEM IN THE SAN JUAN BASIN, NEW MEXICO, USA

Abstract:

This paper integrates outcrop and subsurface investigation of the Upper Cretaceous coastal-marginal-marine system associated with the Gallup Sandstone and its updip and downdip equivalents in the San Juan Basin, northwestern New Mexico, USA. One hundred and forty-three well logs and 95 outcrop measured sections incorporated into three regional outcrop cross sections have been correlated to generate a three-dimensional sequence stratigraphic framework over an area of ∼55,600 km2. The Gallup system consists of offshore to onshore sequences that record Milankovitch frequency shoreline migrations during the ultra-greenhouse of the Turonian Age in the Late Cretaceous Period. The well-log transects define the stratigraphy of the Gallup system in the central part of the basin and broadly match correlations with the previous outcrop-based stratigraphic studies at the basin edges. However, due to the wider spacing of the wells compared to outcrop measured sections, the subsurface sequence stratigraphic framework delimited fewer parasequences than documented in the outcrop sections. Outcrop analysis shows that the Gallup system contains nine facies associations: (1) offshore shelf mudstone, (2) non-deltaic beach to shoreface sandstones, (3) prodelta heterolithics, (4) deltaic sandstones, (5) fluvial sandstones, (6) distributary channel deposits, (7) floodplain deposits, (8) coastal wetlands, and (9) tidal flats. Offshore mudstones dominate the lower part of the Gallup system, whereas shoreface sandstones dominate the upper Gallup Sandstone. Non-marine facies dominate the overlying and updip Torrivio Sandstone, and the coal-bearing Dilco Member of the Crevasse Canyon Formation. Deltaic facies are predominantly found in the northwestern part of the San Juan Basin. In contrast, fair-weather wave-dominated shorefaces and barrier island systems are more dominant in the southeast. This suggests that a fluvial axis was largely confined to the northwest area of the basin and explains the predominance of deltaic systems in the northwest. Paleogeographic reconstructions based on isolith maps and facies analysis indicate that the river system feeding the Gallup delta may have been similar in scale to the Eel River in California, USA, with a drainage area of ∼104 km2. In addition, paleogeographic reconstructions reveal a transition of the shoreline orientation from NWW–SEE to NW–SE trending from the lower Gallup to the upper Gallup interval. This is probably related to increasing river influence in the upper Gallup.