The Tocito Sandstone in a Sequence Stratigraphic Framework: An Example of Landward-Stepping Small-Scale Genetic Sequences
David Valasek, 1995. "The Tocito Sandstone in a Sequence Stratigraphic Framework: An Example of Landward-Stepping Small-Scale Genetic Sequences", Sequence Stratigraphy of Foreland Basin Deposits: Outcrop and Subsurface Examples from the Cretaceous of North America, J.C. Van Wagoner, G.T. Bertram
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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, New Mexico. Tocito sandstones and related marine mudstones represent the seaward half of landward-stepping genetic sequences. Contemporaneous Tocito estuarine, Torrivio Sandstone coarse-grained fluvial, and Dilco Coal Member carbonaceous coastal-plain deposits are deposited landward. Such small-scale genetic sequences differ from progradational sequences. A transgressive unconformity bifurcates temporally equivalent coastal-plain and shelf facies. Tocito palimpsest deposits reflect landward-stepping stratal geometry, while evidence is more cryptic in terrestrial facies. At the seaward point of maximum progradation, the transgressive surface truncates the sequence boundary. Hence, both the sequence boundary unconformity and most, if not all, progradational strata are removed.
Ramp settings are characterized by unconformity creation since accommodation space increases slowly basinward. However, antecedent topography and tectonism may locally increase or decrease the regional accommodation trend. Concomitantly, transgressive processes tend to enhance unconformity creation. Hence, stratal preservation in transgressive ramp settings is reduced. Lowstand wedge deposits may not be preserved in ramp settings with the enhanced erosion. The resultant sediment is dispersed into thin sheet-like sandstone bodies.
Both regressive and transgressive Gallup and Tocito deposits are made up of small-scale genetic sequences that comprise a third-order sequence. Recognition of small-scale sequences is crucial for proper strata correlation and interpretation. Tocito small-scale genetic sequences are a microcosm of large-scale sequences. Landward Torrivio-Dilco deposits overlie a sequence boundary, thin seaward, and are overlain by a transgressive unconformity. Tocito palimpsest deposits overlie a transgressive unconformity which truncated or modified the sequence boundary. The Tocito, Torrivio, and Dilco strata make up a sequence transgressive hemicycle which preserves little to no marine progradational strata. The sequence boundary at, or near, the point of maximum progradation is removed by the transgressive surface of erosion.