PALEOENVIRONMENTAL INTERPRETATION OF THE UPPER CRETACEOUS PT. LOOKOUT SANDSTONE: IMPLICATIONS FOR SHORELINE PROGRADATION AND BASIN TECTONIC HISTORY, SAN JUAN BASIN, NEW MEXICO
Doctor of Philosophy
Detailed stratigraphic and sedimentologic study of outcrop exposures of the Pt. Lookout Fm. and adjacent Mancos and Menefee Fms. of the southeastern San Juan Basin served as the basis of research, whose objectives were to (1) document the small scale facies patterns within the nearshore marine Pt. Lookout Sandstone (2) relate sediment textural characteristics to corresponding environments of deposition, and (3) define factors influencing progradation of the NW-SE trending Pt. Lookout strandline. Five major depositional environments preserved in the progradational sequence are: Offshore Transition Zone (Facies I); Shoreface/Delta Front (Facies II); Foreshore (Facies III); Lower Coastal/Delta Plain (Facies IV); and Upper Coastal/Delta Plain (Facies V). The distribution of these facies reflects interaction of relatively low wave energy, moderate tidal strength, and locally moderate fluvial input along the studied portion of the ancient coastline. Combination of these processes resulted in development of a low wave-energy mainland beach separating an extensive tidal creek complex from the open marine. Where tidal creeks experienced fluvial connection, a dip-aligned delta prograded. Textural parameters (particularly the position of modal population, constancy of the mode, and standard deviation of the sample) successfully distinguish lower shoreface, upper shoreface, delta front/estaurine, and fluvial sandstones. Results suggest that vertical textural progressions may be a useful diagnostic tool in ancient sandstones, including those for which the total range in grain size is narrow and which display diagenetic complications. Regression of the strandline occurred in response to seaward stacking of small, asymmetric fourth-order transgressive-regressive cycles. A model of basinwide progradation predicts broad limits of 17,000 to 66,000 years for fourth-order cycle duration, reflecting probable climatic impact within the basin. Maximum calculated sedimentation rate (0.4m/1000 years) documents relative isolation of the area from tectonically-induced subsidence characteristic of the western U.S. fold-thrust belt. The relative impact of sediment supply-vs-submergence in shaping the strandline migration pattern appears discernible within the geometry of the fourth-order cycles. Although unresolvable in larger cycles, the thickness of fourth-order cycles may separate the factors of sediment supply and submergence controlling shoreline stability through time.