Maars associated with fracture-and/or conduit-controlled aquifers in folded limestones in San Luis Potosí. Aranda-Gómez, J. J.1, Dávila-Harris, P.

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1 Maars associated with fracture-and/or conduit-controlled aquifers in folded limestones in San Luis Potosí Aranda-Gómez, J. J.1, Dávila-Harris, P.2 1 Centro de Geociencias, Campus UNAM Juriquilla, Querétaro, Qro., Mexico Instituto Potosino de Investigación Científica y Tecnológica, Camino a la Presa de San José 2055, Lomas 4a Sección, San Luis Potosí, SLP, 78216, México. 2 Contact: Jorge Aranda (jjag@geociencias.unam.mx) The excursion starts and ends at lobby of Hotel Mision Juriquilla (Queretaro). Departure: November 15 (Saturday) 8.00 AM Arrival: November 17 (Monday) PM Duration: 3 days Objectives: Study the near-vent pyroclastic successions associated with Quaternary maars excavated in folded Mesozoic limestone (Fig. 1). Phreatomagmatic activity was caused by near surface interaction between water contained in the limestone with xenolith-bearing, nephelinitic or hawaiitic magmas. The near vent pyroclastic deposits around these maars are strickingly different from those found around other maars elsewhere, as they are well cemented, steeply dipping in some places (Fig. 2), and massive or diffuse-layered. Some of these craters are nearly elliptical in map view and their long axes are perpendicular to the axial plane of Laramide folds in the limestone (Fig. 3), suggesting that anisotropies in the country rock may have controlled rock fragmentation and crater excavation during phreatomagmatic explosions. Itinerary: Day 1 (Saturday, November 15): Pyroclastic successions around Joya Honda maar and La Joyuela tuff ring in the Ventura volcanic field. An overview of the local structure in the pre-maar rocks of Joya Honda and discussion of its influence on the form of the crater. Drive from Queretaro to the Ventura volcanic fields (Fig. 4) Stop at La Joyuela tuff ring and have a general discussion of regional tectonic setting of the Ventura volcanic field, followed by the inspection of the evidence that the pyroclastic succession exposed around this crater was produced by strombolian activity that was followed by phreatomagmatic blasts. Continue to the southern rim of Joya Honda crater to: 1) study the uneven distribution of pyroclasts around the crater, interpreted as a product of directed blasts (Fig. 1), 2) discuss the evidence of a change from relatively dry phreatomagmatic activity during the initial stages of formation of the maar, which were followed by a significant increase in water in the system, 3) Analysis of Laramide structures exposed on the northern wall of the crater (Fig. 1) and discussion about their probable influence in the development of the elliptical crater (Fig. 3). Night at San Luis Potosí.

2 Day 2 (Sunday, November 16): Maars of the Santo Domingo volcanic field. Influence of a basement structure on the location of intraplate volcanoes. Evidence of intense shearing and metasomatism in the underlying mantle. Drive north from San Luis Potosí to the Santo Domingo volcanic field (Figs. 1 and 5). The first stop will be at the pyroclastic succession associated to the Santo Domingo maar to corroborate the likeness between pyroclastic successions associated to the Ventura and Santo Domingo maars. General discussion about the regional tectonic setting of the Santo Domingo volcanoes, study of tectonite textures and modal metasomatism in the upper mantle xenoliths of the area. Continue to Joya Prieta maar, stop at the western rim of the crater and discussion of the stratigraphic evidence of a dry- to wet- eruptive sequence. Move to Joya de los Contreras maar to visit post-maar vents located on the trace of the inferred ring fault associated to the maar. Drive back to San Luis Potosí to spend the night. Day 3 (Monday, November 17): The Laguna de los Palau maar, a nearly circular crater excavated in pediment gravels overlying folded limestone Drive from San Luis Potosí to the Laguna de los Palau maar (Fig. 6). This volcano was also formed by mafic alkalic magmas. However, the aquifer that interacted with the magma was contained in a gravel deposit derived from erosion of mid-tertiary ignimbrites and Creatceous limestone. The shape of the crater is nearly circular (Fig. 7), thus apparently independent of Laramide structures in the calcareous rocks. Weather conditions: Climate during november is generally sunny and dry although it can get cool at night in in the desert at 1860 above sea level (bring warm clothing). Hikes will not be long, but bring your boots, sunscreen lotion, hat, field gear, and camera. All the volcanoes are located in the southern end of the Chiuahuan desert where thorny bushes and cactus are very abundant; avoid using shorts in the field. Minimum number of participants: 10 Maximum number of participants: 18 Field trip registration deadline : September 1, 2014 Cost: 340 U.S. $. Payment must be made by bank transfer in US currency to: Account name: Maria de Lourdes Godínez-Calderón Account number: Bank: Banamex USA Address of Bank: 2029 Century Park East, 42nd floor Los Angeles, CA 90067, USA ABA: SWIFT Code: CALCUS6L Mexican citizens can pay at the following account: Name: UNAM CENTRO DE GEOCIENCIAS Bank (banco): SANTANDER (MEXICO) S.A. Account (cuenta): Clabe: Reference (Referencia): (Inscripciones MAAR 2014)

3 Cost includes: Transportation, all meals (3 breakfasts, 3 lunch-boxes, 2 dinners, and snacks) and lodging in a shared double room for the entire duration of the excursion, fieldtrip-guide, and other printed materials. Cost does not include: Health insurance and meals and lodging in Querétaro (prior and after the excursion). CANCELLATION: Must be made in writing and received by Jorge Aranda, Field Trip Coordinator, jjag@geociencias.unam.mx by 30 October 2014 to qualify for a 80% refund. NO refunds for cancellation notices received after this date. Refunds will be processed as soon as possible after the meeting.

The tuff breccias are well indurated and display weak or diffuse-layers.

4 Fig. 1. Western and northern walls of Joya Honda. Folded Mesozoic limestone is unconformably overlain by the Quaternary pyroclastic sequence. The tuff breccias are well indurated and display weak or diffuse-layers. Note on the right side of the photo that pyroclastic rocks are missing in the southern wall of the crater. Fig. 2 Steeply dipping, well indurated pyroclastic rocks exposed on the northern rim of Joya Honda.

Based on the geochemistry of the volcanic rocks and the nature of the mantle xenoliths, these clusters are interpreted as two different

5 Fig. 3. Satellite image of Joya Honda. The crater is elliptical and its long axis is perpendicular to the Laramide folds in the country rocks. Fig. 4 Route map from Queretaro to the field area near San Luis Potosí. Note that the volcanoes occur as two separate clusters. Based on the geochemistry of the volcanic rocks and the nature of the mantle xenoliths, these clusters are interpreted as two different intra-plate type volcanic fields. Key: JH = Joya Honda; J = Joyuela, LP = Laguna de los Palau, SD = Santo Domingo maar, JC = Joya de los Contreras, and JP = Joya Prieta.

6 Fig. 5 Route map for the second day. Fig. 6 Route map for the third day.

7 Fig. 7 Satellite image of Laguna de los Palau maar. The crater is excavated in pediment gravels and its shape is circular

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