Lower Cretaceous Alisitos Formation at Punta San Isidro: Coastal sedimentation and volcanism

Abstract

The Lower Cretaceous (Aptian-Albian) Alisitos Formation is well exposed at Punta San Isidro and adjacent sea cliffs on the Pacific shores of Baja California near Eréndira, Mexico. Continuous coastal outcrops define a local stratigraphic succession less than 100 m in total thickness, including repetitious tuff, sandstone and conglomerate units interbedded with discontinuous limestone beds rarely more than 2.5 m thick. The limestone beds are biostromal units that include scattered oysters and/or corals, as well as units dominated by the rudistid bivalve, Caprinuloidea perfecta. Two distinctive conglomerate units are composed of andesite cobbles colonized by encrusting oysters in a quasi rocky-shore setting. One sandstone unit includes abundant fossil wood with tree limbs up to 55 cm long and 5 cm in diameter. Proximal volcanic activity is indicated by a series of dikes that cut through pyroclastic beds and lead to a 10-m thick andesitic flow that caps the succession at Punta San Isidro. Compared with thicker intervals of the Alisitos Formation elsewhere in Baja California that are dominated by andesitic flows and offshore limestone, the Punta San Isidro sequence offers a window on a back-reef environment adjacent to a paleoshore that received pyroclastic lahars from a terrestrial origin or mass flows from shallow submarine explosions. Recovery of marine life and the renewal of a carbonate substrate followed successive episodes of volcanism and massive erosion along an active coastline. This scenario is very different from depositional processes of the Upper Cretaceous (Campanian-Maastrichtian) Rosario Formation that cannibalized and subsequently formed a regional unconformity against tilted Alisitos strata with substantial topographic relief in the Eréndira region.

Early Cretaceous pre-batholith dextral-transpression in the central Baja California peninsula: the record of the La Unión complex, Baja California, Mexico

“…This suite is made up of volcanic rocks with a relatively high proportion of felsic members (>63% SiO 2 ), volcaniclastic rocks, and a regionally extensive shallow water limestone unit (Allison 1974;Gastil et al 1975;Sedlock 2003;Busby et al 2006). Its stratigraphic thickness is estimated as 7.5 km (Allison 1974;Beggs 1984;Almazán-Vázquez 1988;Payne et al 2004), and the oldest reported UPb zircon age is 127 ± 5 Ma (Silver et al 1969) from a tuff sample located near Punta Cabras (locality 4; Figure 1a). This indicates that the age range of the Alisitos Group is highly similar to that of the Santiago Peak Volcanics (e.g.…”

Isotopic chemostratigraphy and biostratigraphy of Lower Cretaceous Alisitos Formation (Punta China section), Baja California, Mexico

“…Palmer is reported to form biostromes in unit 12 of the local stratigraphic section (Payne et al, 2004). This recumbent rudist characterizes the middle Albian Fredericksburg Group in Texas and northern Mexico (Scott and Filkorn, 2007;Scott & Gonzáles León, 1991) and is dated at 107.34-103.92 Ma (Scott, 2014).…”

“…The volcano-sedimentary rocks of the Lower Cretaceous Alisitos Formation are well exposed in the western margin of the Baja California peninsula (Busby, 2004). Previous studies have investigated palaeontology, stratigraphy, and depositional environments of the Alisitos Formation (Allison, 1955a;Allison, 1955b;Allison, 1974;Beggs, 1984;Beltramo, 2003;Busby, 2004;Gastil, Philips, & Allison, 1975;Masse, Beltramo, Martinez-Reyes, & Arnaud-Vanneau, 2007;Payne, Johnson, & Ledesma-Vazquez, 2004;Santillan & Barrera, 1930;Silver, Stehli, & Allen, 1963;Wetmore et al, 2005). Recently, Madhavaraju, Löser, Lee, Lozano-SantaCruz, and Pi-Puig (2016) carried out a geochemical study on limestone samples from the Punta China section, Baja California in order to identify the source of rare earth elements (REEs) and paleo-redox conditions.…”

Síntesis de los trabajos paleobotánicos del Cretácico en México

“…(Helenes, 2000). La Formación Alisitos, aflora en los alrededores del Rancho Alisitos al sureste de El Rosario en Baja California (Santillán y Barrera, 1930), está constituida por rocas volcánicas, volcaniclásticas, metavolcánicas, metasedimentarias y caliza (Payne et al, 2004) y se propuso un ambiente en el depósito someros sublitoral, incluyendo facies arrecifales (Allison, 1963). Fue fechada como Aptiano-Albiano en base a fauna marina (Allison, 1974 (Main, 1950).…”