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Abstract
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Chapter 11
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The Tocito Sandstone
in a  Sequence Stratigraphic Framework: An Example of Landward-Stepping
Small-Scale Genetic Sequences David Valasek
Amoco Production Company
Houston, Texas, U.S.A.
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ABSTRACT
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,
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