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Abstract

In the subsurface of the San Juan Basin, New Mexico, the Coniacian Tocito Sandstone is composed of four sequences (Tocito-1, Tocito-2, Tocito-3 and Tocito-4). Over their extent, each basal sequence boundary is marked by erosion and truncation of underlying strata and onlap by shallower water, typically estuarine strata. Axes of erosion are typically narrow, straight to slightly sinuous and locally join to form tributary-like junctures. These patterns are interpreted to be incised valleys cut by fluvial and estuarine systems during lowstands in relative sea level. The four sequences stack in a backstepping pattern to form a retrogradational sequence set. Toward the outcrop belt (southwest), the sequence boundaries merge to form a composite surface which everywhere separates Tocito strata from underlying Gallup strata.

Sandstone accumulations occur along sinuous lows associated with incised sequence boundaries. Two end-member types of lowstand facies exist: an open-marine facies dominated by marine mudstone with minor thin-bedded and bioturbated sandstone beds and a sand-prone, tidally influenced facies consisting of beds of medium to coarse grained, highly glauconitic sandstone. Tidal indicators include double clay drapes, flaser and lenticular bedding and large- to small-scale sigmoidal and trough cross bedding. Iron-cemented shale rip-up clasts, quartz and phosphatic pebbles, sharks teeth, Inoceramus and oyster shell fragments also characterize the Tocito Sandstone. Ichnofauna are dominated by Thalassinoides, Paleophycus and Planolites burrows with locally abundant robust Ophiomorpha burrows. Bedding and ichnofauna indicate sand deposition within estuaries.

Reactivated basement-involved faults were the dominant influence on Tocito drainage patterns. These northwest-trending basement-involved faults, which were active episodically throughout the Phanerozoic, created linear pathways that acted as catchments for southeast-directed stream and estuarine systems. Reactivation of these structures during the Tocito lowstands led to a reorientation of regional sediment transport directions nearly perpendicular to that of the underlying Gallup shoreline deposits.

The close vertical juxtaposition of erosional sequence boundaries and variably filled incised valleys created abundant stratigraphic traps. Most hydrocarbon traps are dependent on a combination of trapping elements which can include a bend in the valley axis (lateral-trapping clement), valley edges where valley-fill sandstones thin to zero (lateral and updip seals), and truncation of sandstone valley-fill by younger, shale-filled valleys (lateral, updip and top seals). Through time, valleys tend to become broader and shallower and are filled with strata that are more shale-prone and more open marine in character. Hydrocarbon production is chiefly from the older, sand-prone valley systems.

Previous workers viewed the composite erosional surface at the base of the Tocito sequences as a single erosional unconformity which was progressively onlapped through time by beaches, offshore bars and shelf-sand ridges. In these models, hydrocarbon-trapping mechanisms relied on gradual facies changes of the sandy bars into marine shale. The incised-valley model accounts for the lidal and estuarine indicators, complex erosional patterns and hydrocarbon-trapping styles of the Tocito Sandstone.

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