The Yegua sand-bearing genetic unit, of Priabonian (late Middle Eocene age) is recognized as a major genetic sequence in the Gulf Coast, bounded by the Cook Mountain maximum flooding surface (MFS) at the base, and Moodys Branch MFS at the top. Internal cyclicity has been well defined in the Wharton County area (Ewing and Fergeson, 1991; Ewing, 1994) and through the Houston area (Edwards, 1990). If the cyclicity is basin-wide, possibly eustatic in origin, then the cycles should be correlative through the various depositional systems from Texas to Louisiana (Fig. 1). If, however, they are autocyclic in origin, they would not be traceable.

Regional log sections updip of the shelf margin exending from west of Houston, Texas to north of Lafayette, Louisiana have enabled us to correlate most of the marginal-marine Yegua sand-bearing cycles eastward into the Cockfield Formation of Louisiana (Fig. 2). This work is part of a larger project to trace cyclicity across the basin. Also, correlation helps to define sequence context for development and exploration, and can allow knowledge gained in one area to be applied in another.

Important observations include:

• The uppermost Texas cycle below the Moodys Branch MFS (Yegua 10-15, which has been called "Cockfield" in Texas) condenses and disappears into the Moodys Branch as picked in Louisiana. The easternmost sand in this interval occurs in eastern Beauregard Parish (as documented by Lock, 1988).
• The five documented sand-bearing intervals in Allen Parish and eastward are equivalent to the Yegua 20-50 of Central Texas. The Yegua 20 interval is condensed onto the Yegua 30 over a substantial region.
• The lower Yegua cycles (Yegua 60-90) of central Texas (over 330 m, 1100 ft thick) lose sandstone eastward and condense into a limy or marly shale unit (less than 30 m, 100 ft thick) usually called the "Sparta Lime" in central Louisiana. The regional change results in a marked downlap geometry.
• Several of the lower and middle Yegua flooding surfaces are marked by limy stringers in Louisiana. These stringers, usually called "Cook Mountain Lime" are shingled and not correlative.
• In general, the thick shale sequences of Texas become thinner marly shales to the east in Louisiana. The Jackson downlaps entirely and is very thin on the shelf north of Lafayette, Louisiana. The Vicksburg develops a thick marl section in southwest Louisiana, which then thins rapidly eastward.

Figure 1. Trend map showing Yegua/Cockfield depositional systems, the shelf margin position, and the location of the present strike section A-A (in black), sections from Ewing (1994, shaded) and other log sections used or projected.

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Figure 2. Condensed 12-well strike section A-A from Wharton County, Texas to St. Landry Parish, Louisiana, showing flooding surface and sequence boundary correlations in the lower Claiborne Group, Yegua/Cockfield Formation, the Jackson Group and the Vicksburg Formation. Datum is the Moodys Branch MFS. Local sandstone names are also given. GSU = (major) Genetic stratigraphic unit, MFS = Maximum flooding surface, SB = Sequence boundary, UC = Unconformity, FSE = Foundered shelf edge, MB = Moodys Branch, Mp = Massilina pratti, Th = Textularia hockleyensis, Td = Textularia dibollensis.

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• In the center of the Liberty Delta System, extra cycles are identified, and the regional cyclicity is more difficult to trace in parts of the Y-30 and Y-40. There might be an autocyclic component in this area, but most cycles do correlate across the study area.