4.1. THE EL ABRA FORMATION

The older lithostratigraphic unit included in this study is the upper part of the El Abra Formation, deposited on a carbonate platform during the Late Cretaceous. The term El Abra limestone was first applied to the shallow-water carbonate of the Sierra El Abra (cf. ^{Muir, 1936, p. 36}), which is characterized by very thick and poorly defined bedding (Figure 3a) containing a great variety of invertebrates, mostly rudists and other bivalves, as well as gastropods, echinoderms, larger foraminifera and calcareous algae (^{Bonet, 1956}; ^{Alencáster and García-Barrera, 2008}; ^{Buitrón-Sánchez et al., 1995}; ^{Omaña et al., 2012}; ²⁰¹⁹).

Figure 3 View of the outcrops in the Cerro Quemado locality. a) The El Abra Formation outcrop showing the limestone thick bedding; b-d) A panoramic view that shows the contact between the El Abra limestone forming a topographic profile and the overlying “Temazcal unit”; e-f) Pictures of the “Temazcal unit” composed of thick and thin argillaceous limestone layers. 

The larger foraminifera identified from this unit in the CQ succession are Pseudolituonella reicheli Marie, Nezzazata simplex Omara, Daxia cenomana Cuvillier and Szakall, Peneroplis parvus de Castro, Minouxia inflata Marie, Cuneolina parva Henson, C. pavonia d’Orbigny, Nezzazatinella picardi (Henson) and miliolids. We assume that it corresponds to the Pseudolituonella reicheli Assemblage Zone.

The microfacies of the interval is a benthic foraminiferal packstone/grainstone, well sorted, sub-rounded micritic intraclasts are associated with peloids and diverse benthic foraminifera association, including miliolids and Peneroplis, Daxia, Pseudolituonella, fragments of mollusk are common.

These components have a chaotic orientation and are densely packed. Some bioclasts exhibit an irregular micrite envelope. Sparry calcite cement fills inter- and intragranular and pores. (Figure 4a). It could be compared to Standard Microfacies Type (SMF 18) of ^{Flügel (2010)}, indicating a shallow-water marine environment.

Figure 4 Microfacies in the boundary between the El Abra Formation and the “Temazcal unit”, showing the lithological and micropaleontological differences. a) Benthic foraminiferal packstone/grainstone with benthic foraminifera from the upper part of the El Abra Formation, Sample CQ- 00a-1; b) Benthic foraminiferal packstone-grainstone with Peneroplis, Nezzazatinella, Cuneolina from the uppermost part of the El Abra limestone, Sample CQ-001(1); c-d) Mixture of benthic (B) and planktic foraminifera (P) that extends from the El Abra Formation into the overlying unit; c = Sample CQ-01(1); d = Sample CQ-01(2); e) Wackestone with planktic foraminifera characteristic of the “Temazcal unit”; e = Sample CQ-03 (2); f = Sample CQ-02b (7). Scale bar 100 µm in a - d. Scale bar 200 µm in e. 

4.2. THE “TEMAZCAL UNIT”

We propose the name “Temazcal unit” as an informal name for an outcropping located at the Cerro Quemado (22º 23’ 24.65” N; 100º 30’ 29.69” W), municipality of Armadillo de los Infante in the state of San Luis Potosí. This lithological unit consists of thin and thick layers of gray to green silty limestone that has weathered to a reddish color (Figure 3e, 3f). The base of the unit unconformably overlies the El Abra Formation and there are no other marine layers covering it in the area. In the contact between the two units, we observed evidence of an ancient sub-aerial exposure that had allowed the development of significant topographic relief as result of erosion (Figure 3b to 3d).

The foraminiferal association in the uppermost part of the El Abra Formation, underlying the Temazcal unit, is composed mostly of Nezzatinella picardi (Henson), Cuneolina parva Henson, and miliolids (Nezzazatinella picardi Interval Zone) (Figure 4a) of mid-late Cenomanian age (^{Omaña et al., 2019}), the microfacies is a foraminiferal packstone/grainstone as the above described, but with a different benthic foraminiferal assemblage. It could be compared to Standard Microfacies Type (SMF 18) of ^{Flügel (2010)}, indicating a shallow-water marine environment (Figure 4b).

Upward, a mixture of benthic and planktic foraminifera is recorded. In the shift towards the Temazcal unit, the microfacies is an intraclastic-bioclastic wackstone/packstone benthic foraminiferal, cuneolinids, nezzazatinellids (Figure 4c), observed together with planktic foraminifera as globotruncanids and rugoglobigerinids (Figure 4d). Subrounded intraclasts containing peloids, are also present. These components are embedded within a dark-colored micritic matrix commonly neomorphosed to microspar. Some intraskeletal pores are filled with sparry calcite. Bioclasts are usually silicified. Over it, a planktic foraminiferal wackstone/packstone, these microfossils are the only bioclast present in this microfacies. These components are embedded within a dark-colored micritic matrix neomorphosed to microspar. A distinct feature is the occurrence of planar lamination. Laminae are differentiated by the content of grains. Usually, bioclasts are oriented parallel to the lamination.

We infer a pelagic deep-water basinal facies (FZ 1) of ^{Wilson (1975)} or SMF 3 according to ^{Flügel (2010)} (Figure 4e, 4f).

Within the “Temazcal unit,” the planktic foraminiferal association is reported and illustrated for the first time in this area; it consists of Abathomphalus mayaroensis (Bolli), A. intermedius (Bolli), Plummerita hantkeninoides (Brönnimann), Trinitella scotti (Brönnimann), Kassabiana falsocalcarata (Kerdany and Abdelsalam), Planoglobulina acervulinoides (Egger), Globotruncanita conica (White), G. stuarti (de Lapparent), G. stuartiformis (Dalbiez), Contusotruncana walfishensis (Todd), Globotruncana arca (Cushman), G. bulloides Vogler, G. hilli Pessagno, G. falsostuarti Sigal, Rugoglobigerina macrocephala (Brönnimann), R. hexacamerata (Brönnimann), R. milamensis Smith and Pessagno, Planoglobulina carseyae (Plummer) (Figures 5 to 7).

Figure 5 Late Maastrichtian Planktic foraminifera from the “Temazcal unit”. a) Kassabiana falsocalcarata (Kerdany and Abdelsalam) transversal section, Sample CQ-02b (7); b-c) Plummerita hantkeninoides (Brönnimann) transversal section, Sample CQ-02a (4), c) axial section, Sample CQ-02b (5); d) Rugoglobigerina macrocephala (Brönnimann) transversal section, Sample CQ-02b (5); e) R. milamensis Smith and Pessagno axial section, Sample CQ-02b (5); f) Plummerita cf. P. reicheli (Brönnimann) transversal section, Sample CQ-02b (7); g) Planoglobulina carseyae (Plummer) axial section, Sample CQ-02b (8); h) Pseudotextuaria elegans (Rzehak) axial section, Sample CQ02b (7); i) Abathomphalus intermedius (Bolli) axial section, Sample CQ-02a (5); j) Globotruncanita conica (White) axial section, Sample CQ-02b (7); k) Macroglobigerinelloides prairiehillensis (Pessagno) axial section, Sample CQ-02b (5). Scale bar 200 µm. 

Figure 6 Late Maastrichtian planktic foraminifera from the “Temazcal unit”. a) Globotruncana falsostuarti Sigal axial section, Sample CQ-03 (3); b) Contusotruncana walfischensis (Todd) axial section, Sample CQ-03 (2); c) G. hilli Pessagno axial section, Sample CQ-02b (10); d) Globotruncanita stuartiformis (Dalbiez) axial section, Sample CQ-02b (6); e) Globotruncana arca (Cushman) axial section, Sample CQ-03 (4); f) G. bulloides Vogler axial section, Sample CQ-02b (2); g) Abathomphalus mayaroensis (Bolli) axial section, Sample CQ-02b (2); h) G. aegyptiaca Nakkady axial section, Sample CQ-02b (3); i) Trinitella scotti Brönnimann transversal section, Sample CQ-03 (3). Scale bar 200 µm. 

Figure 7 Late Maastrichtian planktic foraminifera from the “Temazcal unit”. a - g) Planoglobulina acervulinoides (Egger),transversal section, Sample CQ-02b (4); h) P. multicamerata (de Klasz) transversal section, Sample CQ-03 (1). Scale bar 200 µm. 

Based on the planktic foraminiferal association, the age assigned to this succession is late Maastrichtian (Abathomphalus mayaroensis Interval Zone) according to the zonal schemes for tropical regions (^{Caron, 1985}; ^{Premoli-Silva and Sliter, 1995}; ^{Robaszynski and Caron 1995}; ^{Premoli-Silva and Verga, 2004}). The age of this unit is correlated with the upper part of the Méndez Formation, cropping out in the eastern part of the VSLPP.

Evidence of an unconformity in the upper part of the El Abra Formation has been widely recognized. The unit has been overlain by units of distinct ages and recorded in different localities, so a controversy has existed since ^{Kellum (1930, p. 78)} indicated that the upper part of the Méndez Formation overlies the El Abra limestone, noting also that possibly the San Felipe Formation and the basal Méndez are absent in a section to the east and southeast of the Sierra El Abra. This view is confirmed by ^{Muir (1936, p. 62)}, who suggests unconformable contacts between the El Abra and the San Felipe and Méndez formations. ^{Bonet (1952)} showed that the Méndez Formation crops out along the escarpment of the Sierra El Abra rests unconformably on the limestone of the El Abra Formation. Later, ^{Pessagno (1969)} concluded that the Campanian Méndez strata are in contact with El Abra Formation due to thrust faulting at southwest flank of Sierra El Abra northeast of Antiguo Morelos, Tamaulipas. ^{Carrillo-Bravo (1971, p. 36-42)} stated that in the Sierra de El Abra northeast of the Gómez Farias region, the Turonian Agua Nueva and San Felipe Formations disconformably overlie the El Abra Formation, while east of Jaumave it is covered by the San Felipe and the Méndez formations and in the Miquihuana area the El Abra Formation is capped by the Méndez Formation of Campanian-Maastrichtian age. All these localities are in the state of Tamaulipas. ^{Aguayo and Kanamori (1976)} proposed the Tamuin Member of the mid-part of the Méndez Formation, suggesting that it overlies the El Abra Formation in angular unconformity.

^{Smith (1986, p. 148)} stated that “during the Late Cretaceous certain areas were subaerially exposed on the VSLPP. The maximum hiatus recorded on the platform is where the Méndez Formation overlies El Abra, however the duration of the hiatus could be estimated depending on which strata overlie El Abra Formation at a given location.”

^{Basáñez et al. (1993)} noted that various formations, such as the Agua Nueva, San Felipe and Méndez, covered the El Abra Limestone at the eastern part of the VSLPP. It is important to point out that although the El Abra unconformity has been recognized on the VSLPP platform for a long time, the present study has enabled the magnitude of the hiatus between the El Abra Formation and the “Temazcal Unit” in the Cerro Quemado locality to be established. The gap would be from the early Turonian deposit up to the early Maastrichtian, based both on the stratigraphic relationships and on the exhaustive micropaleontologic and petrographic studies.

Since the Turonian-early Maastrichtian, the area occupied by the Valles-San Luis Potosí Platform was located within the foreland zone and involved in the contractive deformation of the Mexican Orogeny (^{Juárez-Arriaga et al., 2019}). As an uplifted basement block since the Late Jurassic, during the Early Cretaceous this area was flooded by a shallow sea with a proliferation of reefs and lagoons, surrounded by relatively deeper basins. These basinal areas have been grouped as the Mexican Foreland Basin (Juarez-Arriaga et al., 2016, 2019). In such a scenario, the Valles San Luis Potosí was a part of the forebulge zone of the Mexican orogen in front of the foreland basin and reached an elevated position by tectonic effects, experiencing marine regression from the Turonian, with only ephemeral transgressions towards the late Maastrichtian as documented at least for the area of our study.