ABSTRACT

Three principal depositional systems are defined within the Sparta Formation of Texas using surface and subsurface data: high-constructive delta system in east Texas, strandplain/barrier-bar system in central Texas, and high-destructive, wave-dominated delta system in south Texas.

Principal facies constituents of the high-constructive delta include upper delta plain in outcrop and lower delta plain. delta front, and prodelta in subsurface. Five major deltaic lobes in the Sparta Formation are similar to various lobes of the Eocene Queen City Formation, lower Wilcox Group, Jackson Group, and Yegua Formation of Texas. The Sparta high-constructive delta system is present from Fayette and Colorado counties in Texas, eastward into Louisiana, Mississippi, and Arkansas.

The Sparta strandplain/barrier-bar system of central Texas is basically composed of a single multistory barrier bar unit. It was constructed with sand transported along strike by longshore currents from reworked sediments of the high-constructive delta system in east Texas. This system extends from Fayette and Colorado counties westward to Atascosa and Live Oak counties. A Holocene analog is the Texas barrier island system. Eocene analogs are the strandplain/barrier-bar system of the Eocene Jackson Group. Yegua Formation, lower Wilcox Group, and Queen City Formation.

A high-destructive delta system in south Texas is composed essentially of coastal barriers and associated lagoonal facies in outcrop and coastal barrier, lagoon, and prodelta shelf facies in the subsurface. This wave-dominated delta system is present from Atascosa and Live Oak counties southward to the Rio Grande, and it extends into northern Mexico. Eocene analogs occur in the south Texas Wilcox Group, Yegua Formation, and Queen City Formation.

Oil and gas have not been found in the Sparta Formation, in part because little growth faulting was associated with the thin Sparta delta-front sandstone and prodelta shale facies. Water chemistry variations are closely related to depositional systems within the Sparta Formation. A bicarbonate province is related to updip areas (major fluvial influence) of the high-constructive delta system of east Texas; a sulfate province occurs in updip areas (barrier-bar/lagoon influence) associated with the high-destructive delta system of south Texas and central Texas; a chloride province is associated with downdip marine sandstone facies of barrier and deltaic origin. Flushing by fresh water has quantitatively but not qualitatively altered the initial water distribution within the various Sparta sand facies.