Depositional Systems in the Nacatoch Formation (Upper Cretaceous), Northeast Texas and Southwest Arkansas

The Nacatoch Formation of the East Texas Basin is the middle formation of the Navarro Group and consists of marine sandstones and mudstones derived largely from source areas to the northwest, north, and northeast of the East Texas Embayment. Terrigenous clastics were supplied to the Nacatoch Basin by a major northeastern dispersal system originating in southwest Arkansas. Three minor fluvial-delta systems contributed sediment in southern Red River, Delta, and Hunt Counties, Texas. Five facies are recognized in Nacatoch outcrops in southwest Arkansas: tidal-flat, tidal-channel, tidal-inlet- associated, shoreface, and shelf facies. In northeast Texas, a delta sequence occurs in south-central Hunt County, and shelf sandstones and mudstones are present in Navarro and Kaufman Counties. The lateral association of deltaic deposits and tidal-flat sequences, together with the type, scale, and distribution pattern of inferred tide-produced structures, suggests that tides within the upper microtidal to lower mesotidal range (3 to 8 ft; 1 to 2.5 m) occurred in the East Texas and North Louisiana Embayments during deposition of the Nacatoch Formation. The Nacatoch Formation in the East Texas Basin is restricted to the northern and western parts of the basin. The sandstone bodies trend mainly northeast to southwest in the northern part of the basin and north to south along the western margin. In the southern half of the basin, the Nacatoch Formation consists of mudstones. In the subsurface of the East Texas Basin, the Nacatoch Formation can generally be subdivided into nearshore and shelf deposits. Nearshore sequences include deltaic deposits in the north and the northwest parts of the basin that are located downdip from surface exposures of the same facies. Two thick net-sand axes, oriented perpendicularly to the outcrop belt, extend southward into the basin. Orientation of these sand axes changes abruptly to become parallel within the dominant northeast-southwest trend, suggesting that the delta was dominated by tides and waves. It is inferred that interdeltaic areas were sites of short barrier islands, broad tidal inlets with associated tidal deltas, and tidal flats. Offshore deposits can be arbitrarily divided into a lower and an upper sandstone sequence separated by 50 to 100 ft (16.6 to 33.3 m) of marine mudstone. Sandstone bodies of the lower sequence are elongate, exhibit gradational lower boundaries and abrupt upper contacts, and grade laterally into muddy sandstones and mudstones. Sandstones composing these depositional sequences are well sorted, calcitic, glauconitic, fine to medium grained, and contain shell fragments. The sandstone bodies are interpreted to be offshore bars, which have a geometry derived primarily from tidal currents. Sandstones of the upper sequence compose a fairly continuous sheet sand; textures and composition are similar to sandstones of the lower sequence. Tectonism, coincident with deposition, controlled local sandstone distribution patterns. Development of rim synclines concomitant with salt dome growth considerably affected the thickness and distribution of the Nacatoch Formation; for example, thick Nacatoch sections exist around Haynesville salt dome in Wood County, Texas. Other piercement domes associated with salt withdrawal basins that were active during Nacatoch deposition are Steen, Mt. Sylvan, East Tyler, Brooks, and Bethel. Few sandstones occur in the Nacatoch Formation in the southern part of the East Texas Basin. These thin, laterally discontinuous sandstone bodies do not threaten the hydrologic integrity of salt domes now being investigated to determine their feasibility for nuclear waste storage. Sandstones within the Nacatoch Formation in the East Texas Basin are important shallow oil and gas reservoirs. Hydrocarbon reservoirs from the Nacatoch Formation are restricted to the shelf-sand facies. However, hydrocarbon entrapment appears to be more a function of structural closure than of depositional facies. Hydrocarbons are produced from Nacatoch fields developed over the Van salt dome in Van Zandt County and along the Mexia-Talco fault system trend near the western margin of the basin.