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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 49 (1965)

Issue: 3. (March)

First Page: 349

Last Page: 349

Title: Cretaceous Strike-Valley Sandstones, Northwestern New Mexico: ABSTRACT

Author(s): Donald G. McCubbin

Article Type: Meeting abstract


Cretaceous sandstone bodies, which are the principal oil reservoirs in the San Juan Basin, have been interpreted as offshore bars related to deposition of the regressive Gallup Sandstone. A detailed outcrop and subsurface study in the northwestern San Juan Basin indicates that these sandstone bodies are strike-valley sandstones in the transgressive marine "basal Niobrara" unit which rests unconformably on the Gallup Sandstone and older units.

Paleotopography of the pre-Niobrara unconformity consisted of northwest-southeast trending cuestas and intervening strike valleys with local relief of over 100 feet. Cuestas and valleys are related to the subcrop of alternating resistant and non-resistant units in the truncated sequence. Elongate, lenticular sandstone bodies overlying the unconformity occur in strike valleys on the northeast side of cuesta scarps. Individual sandstone bodies, with a maximum thickness of about 50 feet, lap out to the southwest against the cuestas and thin to the northeast by facies change to shale. Younger sandstone bodies extend progressively farther to the southwest.

Basal Niobrara sandstones are fine to coarse grained and glauconitic, and contain marine macrofossils. Interbedded and laterally equivalent shales also contain marine fossils, including both benthonic and pelagic forms. The sandstones are characterized by broadly lenticular sets (up to 6 feet thick) of high-angle cross-stratification. Measurements of cross-stratification dip directions at 20 localities indicate transport by currents flowing generally southeastward.

These sandstones are best interpreted as nearshore marine sands deposited in strike valleys on the seaward side of cuesta scarps during a general transgression to the southwest. During the transgression, cuesta ridges acted to stabilize temporarily the position of the shoreline, permitting accumulation of sand nearshore while clays were deposited farther offshore to the northeast. Transport of sand was largely in the form of underwater dunes migrating alongshore to the southeast.

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Copyright 1997 American Association of Petroleum Geologists