Published 1995 as part of Memoir 64
Copyright © 1995 The American Association of Petroleum Geologists.  All Rights Reserved.
 

Recognition of depositional sequences in the Upper Cretaceous (of early to middle Coniacian age) Tocito Sandstone permits determination of genetic relationships of coeval shelf shoreface alluvial and carbonaceous strata. Stratal geometry, lithologic character, and facies change with position in the stratal architecture. Interpretation of Tocito Sandstone depositional environment is controversial since the Tocito is isolated by unconformities or encased in marine mudstones. The sequence stratigraphic framework is just as controversial since strata are used to interpret bounding surfaces. The Gallup-Tocito system was deposited in a ramp setting and is characterized by thin strata and amalgamated unconformities.

The Tocito consists of interbedded coarse- to medium-grained fossiliferous sandstones and mudstones which overlie a regional unconformity in the northwestern San Juan basin, New Mexico. The Tocito depositional environment controversy can be cleared by identifying two facies based on sandstone characteristics. Homogenous, horizontally bedded, coarse-grained sandstones which make up the bulk of Tocito are interpreted as palimpsest shelf deposits. Mudstones and condensed sections overlie palimpsest deposits. Heterolithic interbedded sandstones and mudstones with both seaward- and landward-dipping clinoforms are interpreted as estuarine deposits. Estuarine facies, truncated by unconformities overlain by palimpsest facies, occur seaward of the Gallup Sandstone pinch-out and make up a small part of the Tocito. Both facies suites are present at only a few locations. Most Tocito facies disputes are based on interpretations of strata at locations where one facies, but not the other, is present. Further confusion occurs since there are several Gallup Sandstone seaward-stepping, very small-scale genetic sequences that are often misidentified as Tocito sandstones.

Twelve separate transgressive small-scale genetic sequences are identified and correlated from continuous outcrops west and southwest of Shiprock,