ABSTRACT

The Frio Formation of the northern Mustang Island Texas State waters area was deposited in an active growth-faulted basin on the flanks of the Norias delta system. Biostratigraphic analysis of several recently drilled wells and reinterpretation of older biostratigraphic well reports (Fig. 1) suggest that several local biostratigraphic markers may be more reliably used in conjunction with well log and seismic correlations than the more recognized regional datums (Fig. 2). Older biostratigraphic reports usually recognize only two regional markers, Marginulina texana and Bolivina mexicana. Ecologically, Marginulina texana is most reliable updip (inner to middle neritic), whereas Bolivina mexicana is limited to outer neritic or deeper environments.

Within the expanded Marginulina texana section, several local foraminiferal events are useful for enhanced correlation (Breard et al., 1996). Two closely-spaced extinctions or last appearance datum (LADs), Nonion 2 and Eponides vicksburgensis, possibly representing a condensed zone, are correlative with a transgressive sand that ties to a strong seismic event mappable throughout the study area. Another marker, Discorbis E, is correlative between the updip wells and may constrain the Bolivina mexicana equivalent section in environments too shallow to support that marker.

The lower expanded Marginulina texana or Bolivina mexicana sands become progressively shallower through deltaic influence, resulting in lower faunal diversityand fewer correlatable assemblages (Fig. 3). Several acmes of Cibicides hazzardi and a zone of common Textularia spp. appear correlative within the deltaic sands. This study illustrates the value of local foraminiferal events for correlation of expanded stratigraphic sections along depositional strike.

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Figure 1. Location map--northern Mustang Island. From Seni et al. (1997)

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Figure 2. Local biostratigraphic chart, Upper Frio (Oligocene), northern Mustang Island area, Texas State Waters.

Figure 3. Observed foraminiferal environmental indicator taxa, Upper Frio (Oligocene), northern Mustang Island area, Texas State Waters.

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