Point Lookout Sandstone

Point Lookout Sandstone
Name	Point Lookout Sandstone
Type	Geological formation
Period	Campanian
Priod	Cretaceous
Lithology	Sandstone, shale, conglomerate
Namedfor	Point Lookout
Region	New Mexico, Colorado, Arizona
Country	United States

Point Lookout Sandstone is a Campanian-age Cretaceous sandstone unit widely recognized in the Western Interior Seaway margin of the United States Southwest. It forms a conspicuous cliff-forming succession that records transgressive–regressive cycles along the ancient continental margin and underlies the coal-bearing Menefee Formation and overlies the marine Crevasse Canyon Formation. The unit is notable for sandstone-dominated facies, fluvial and shoreline fossils, and importance for regional stratigraphic correlations used by petroleum and water-resource studies.

Description and Lithology

The Point Lookout Sandstone is primarily a medium- to coarse-grained, well-sorted, quartzose sandstone with interbeds of feldspathic sandstone, siltstone, and shale, and local pebble conglomerate matrices that display cross-bedding and planar-lamination features characteristic of high-energy shoreface and deltaic systems; key exposures show meter-scale foreset cross-strata similar to those described in studies of the Monterey Formation, Navajo Sandstone, and Tuscaloosa Formation. Cementation ranges from calcite to silica and iron oxide, producing rusty-weathering outcrops comparable to Entrada Sandstone and Dakota Sandstone surfaces. Bedding surfaces commonly exhibit burrows, rooted horizons, and planar beds that link sedimentary structures to those documented in the Fox Hills Formation and Mancos Shale.

Stratigraphy and Age

Stratigraphically, the Point Lookout Sandstone occupies the upper part of the Mesaverde Group in classic sections and is constrained to the late Cretaceous Campanian by biostratigraphic ties to ammonite and inoceramid faunas and by palynology comparable to assemblages used in correlations with the Pierre Shale and Niobrara Formation. It forms a regional marker bed that is conformable to locally disconformable with underlying and overlying units, echoing sequence boundaries recognized in the Zuni Basin, San Juan Basin, and Paradox Basin. Lithostratigraphic comparisons have linked type sections to coeval units in the Raton Basin and the Los Pinos Mountains.

Geographic Distribution and Type Locality

Exposures of the Point Lookout Sandstone are most extensive in northwestern New Mexico, southwestern Colorado, and northeastern Arizona, with classic cliffs at the type locality near Point Lookout on the San Juan River corridor and along road cuts adjacent to U.S. Route 491 and Interstate 25. The formation extends into outliers within the Chaco Canyon area and is mapped in the Four Corners region, with lateral facies changes recorded toward the Western Interior Basin margins and along paleoshorelines leading to the Gulf of Mexico embayment.

Depositional Environment and Paleogeography

The unit represents prograding shoreface, deltaic, and estuarine deposits formed at the western margin of the Western Interior Seaway during a regressive phase similar to depositional regimes described for the Shinumo Sandstone and Ferron Sandstone. Sedimentary structures, paleocurrent indicators, and clast provenance studies indicate sediment supply from uplifted Ancestral Rocky Mountains sources and drainage systems linked to paleorivers analogous to those documented in the Brazos River catchment reconstructions, with repeated transgression–regression cycles comparable to sea-level fluctuations recorded in the Greenhorn Limestone and Carlile Shale sequences. Paleogeographic reconstructions place the formation at mid-latitudes adjacent to shallow epicontinental seas, where coastal plains, barrier islands, and tidal channels formed dynamic depositional complexes.

Fossil Content and Paleontology

Fossils in the Point Lookout Sandstone are generally sparse but include trace fossils, plant debris, and occasional vertebrate and invertebrate remains; reported ichnofossils and benthic traces parallel trace assemblages from the Dakota Formation and the Mancos Shale. Plant macrofossils and coalified wood tie the unit to terrestrial floras correlated with Campanian palynofloras used in studies of the Hell Creek Formation and Fort Union Formation. Marine incursions have yielded molluscan and ammonite fragments that aid correlations with the Pierre Shale faunal zones, and isolated dinosaur teeth and bone fragments reported from channel fills align with finds in the Kirtland Formation and Fruitland Formation.

Economic Uses and Engineering Properties

The Point Lookout Sandstone serves as a prominent reservoir analog for shallow hydrocarbon and groundwater studies and has been evaluated for porosity, permeability, and fracturing potential in regional energy assessments similar to work on the Mesaverde Group and Mancos Shale. Its competent, cliff-forming sandstone is commonly used as building stone and aggregate in local infrastructure projects, akin to uses of the Sawatch Sandstone and Entrada Sandstone. Engineering challenges include slope stability and differential weathering where the sandstone overlies weaker shale units, producing hazards documented in civil works near Durango, Colorado and Farmington, New Mexico.

Research History and Notable Studies

The formation was first described in regional mapping campaigns associated with early 20th-century surveys by the United States Geological Survey and later refined in Mesaverde stratigraphic syntheses by researchers affiliated with New Mexico Bureau of Geology and Mineral Resources, Colorado Geological Survey, and university teams at University of New Mexico and Colorado School of Mines. Notable studies have addressed sequence stratigraphy, petrography, and basin analysis and appear in comparative frameworks alongside investigations of the Western Interior Basin stratigraphy, the Sevier orogeny-related provenance, and sediment dispersal systems documented in the literature of AAPG and Geological Society of America conferences. Recent work integrates detrital zircon geochronology and seismic-scale correlation methods as applied to adjacent units such as the Menefee Formation and Pictured Cliffs Sandstone.

Category:Cretaceous geology of New Mexico