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Abstract

In the subsurface of the San Juan basin, New Mexico, the Coniacian Tocito Sandstone is composed of four high-frequency sequences. Basal sequence boundaries are marked by erosional truncation of underlying strata and have mappable axes of erosion which are typically narrow and linear. These erosional features are interpreted to be incised valleys cut by fluvial and estuarine processes during lowstands in relative sea level. Two end-member facies are found in the valley fills: (1) a low-energy mudstone-dominated facies with thin-bedded and bioturbated sandstones, and (2) a high-energy facies chiefly consisting of medium- to coarse-grained glauconitic sandstone. Tidal indicators in this facies include double clay drapes, flaser and lenticular bedding, and large- to small-scale sigmoidal cross-bedding. Iron-cemented shale rip-up clasts, quartz and phosphatic pebbles, sharks’ teeth, and detrital fragments of Inoceramus and oyster shells also characterize the Tocito Sandstone. Paleophycus, Thalassinoid.es, and Planolites burrows with locally abundant Ophiomorpha burrows typify the low-diversity, low- to moderate-abundance trace-fossil suite. The combination of lithofacies, sedimentary structures, and ichnofauna indicates lowstand deposition within tide-dominated estuarine systems. The sandstone-prone fills are considered proximal facies connected to fluvial systems, whereas the shalier facies are interpreted to be more distal fills laid down as the estuaries expanded to form large embayments.

Valley systems show a sequential change in geometry and fill type in vertical profile: older valleys are narrow and sandstone filled and give way to broader systems which are filled with strata which are more shale-prone.

The result is a retrogradational stacking of the sequences or a transgressive sequence set. This stacking pattern, coupled with the erosional juxtaposition of the valley fills, created many stratigraphic traps for hydrocarbons. The two principal trap types are: (1) bends in valley axes where valley-fill sandstones thin laterally to zero against underlying marine mudstones, and (2) truncation of valley-fill sandstones by younger, shale-filled valleys. Hydrocarbons are produced from the older, sandstone-prone valleys with top and lateral seals provided by the overlying, younger shale-filled valleys.

The strongly parallel and southeast-flowing drainage pattern found in the Tocito lowstands parallels the basement-involved fault trend. A phase of structural reactivation is inferred to have accompanied Tocito deposition and caused sediment dispersal patterns to be nearly perpendicular to the northeast prograding Gallup highstand system.

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