Figure Captions 

	Figure 1. Location map of the Macuspana Basin, showing the wells studied and the lithostratigraphic column of the basin.
	Figure 2. Neogene calcareous nannofossil, planktonic foraminiferal, and composite calcareous microfossil biostratigraphy for the Macuspana Basin.
	Figure 3. Chronostratigraphic charts of the wells studied for the Macuspana Basin.

 

Setting 

The Macuspana Basin is located in the eastern termination of the southern Gulf of Mexico physiographic province (Figure 1). It is bordered by the Sierra Chiapas fold belt to the south, the Yucatan platform to the east, and the Reforma-Akal uplift to the west. This study presents the results of a taxonomic, biostratigraphic, paleobathymetric, and paleoenvironmental study, using calcareous nannofossils and foraminifera in two exploratory wells in southeastern part of Mexico. The onshore part of Macuspana Basin includes shallow marine clastic sediments with few microfossil markers within a complex structural setting. By using two different microfossil groups we obtained results with more resolution and greater reliability.

 

Biostratigraphy 

A detailed, quantitative calcareous nannofossil analysis was performed on samples from two sections in this basin. The main objective was to document the distribution patterns of calcareous nannofossils to identify and accurately date the recognized events through the Neogene interval. In the sections studied, the calcareous nannofossil assemblages are generally common, well preserved, and moderately diverse. The quantitative analyses of 223 cuttings samples allowed the recognition of abundance patterns of the observed taxa, and the assignments of ages using the chronostratigraphic time scale of Berggren et al. (1995). 

Distribution of planktonic foraminifera in 78 ditch cuttings samples and sidewall cores from Neogene the interval is documented. The foraminiferal database and geophysical logs were provided by the Paleontology and Petrography Laboratory, PEMEX. We integrated calcareous nannofossil biostratigraphy and planktonic foraminifera biostratigraphic events to determine a composite calcareous microfossil biostratigraphy (Figure 2). 

Benthonic foraminiferal biofacies provide information about marine conditions in the investigated section during the Neogene. Some genera of calcareous nannofossils allow interpretation of shallow-water, inner neritic environment (Helicosphaera, Braarudosphaera) and oceanic conditions of deep water (Discoaster, Minylitha). Distribution of benthonic foraminifera is the basis for the paleobathymetric reconstruction in the sections of the basin. 

The quantitative information we gathered allowed us to recognize transgressive-regressive cycles. Based on our data we propose the presence of the following candidates for third order stratigraphic sequences: in well A, we identified the cycles TB-2.6, TB-3.1 and TB-3.5; while in well B we identified the cycles TB-3.1, TB-3.2, TB-3.3 and TB-3.6 (Figure 3). Our interpretation indicates unconformities in the late Miocene to Pliocene interval. In well A, the unconformity is found in between the TB-3.1 and TB-3.5 cycles; whereas in well B, the unconformity identified is located between the TB-3.3 and TB-3.6 cycles.

 

Conclusions 

• Three candidate sequences in well A were recognized. The age of this section is middle Miocene to early Pliocene.

• Four candidate sequences in well B were recognized. The age of this section is Miocene to Pleistocene.

• In well A, the section was deposited in mainly transitional through outer neritic environments, whereas, in well B the sediments were deposited in upper bathyal environments.

• The biostratigraphic data indicate the presence of an unconformity between the Miocene and Pliocene sedimentary rocks.

 

Acknowledgments 

This research was financially supported by C.I.C.E.S.E., Gulf Coast Association of Geological Societies for student research grant 2004, and specially to PEMEX for the information and investigation material.