Regional and detailed structural-stratigraphic studies of the Oligocene in the Burgos basin (northeast Mexico) point up the great significance of many growth and postdepositional faults. Several unconformities, together with abrupt facies changes, add to the complexity of stratigraphic and structural analysis.

An interdisciplinary (team) approach was used, involving the construction of both regional and detailed electric-log correlation grids. Such correlations were supplemented by petrologic, micropaleontologic, and seismic studies.

Within a selected pilot area, the lower Oligocene (Vicksburg) sedimentary rocks consist primarily of marine shales interbedded with marine lenticular sandstones. The middle Oligocene (Frio), composed predominantly of nonmarine sandstones interbedded with shale, has a maximum overall thickness of 2,000 m. The basal middle Oligocene contains brackish-water sandstones and shales. In the western part of the area, the nonmarine Norma conglomerate lies unconformably on truncated beds of the lower Oligocene marine sequence. This conglomerate thins abruptly in the eastern third of the pilot area and grades into a thick sequence of nonmarine (Frio) sandstones and shales, on the downthrown side of the McAllen growth fault. The Norma conglomerate is absent north of the Rio Grande River.

The upper Oligocene (Anahuac) is a westward-thinning wedge of marine sandstones and shales, with a maximum observed thickness of 1,000 m. Although in the Gulf Coast of Texas, the Anahuac generally is considered to be of late Oligocene age, the lowest part in northeastern Mexico appears to be of late middle Oligocene age. The basal Marginulina zone was deposited under marine transgressive conditions and interfingers with Frio sediments. Only the thinnest (upper) part of this zone overlies nonmarine Frio.

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