Revista Mexicana de Ciencias Geológicas,Stratigraphy 2006, v. and 23, structurenúm, 1, 2006, of the p. Altar 1-22 basin of NW Sonora 1 Stratigraphy and structure of the Altar basin of NW Sonora: Implications for the history of the Colorado River delta and the Salton trough Martín Pacheco1, Arturo Martín-Barajas1,*, Wilfred Elders2, Juan Manuel Espinosa-Cardeña1, Javier Helenes1, and Alberto Segura3 1 División de Ciencias de la Tierra, CICESE, Km 107 Carr. Tijuana-Ensenada, Ensenada, Baja California, Mexico. 2 Department of Earth Sciences, University of California, Riverside, CA 92521, USA. 3Activo Regional de Exploración, Región Norte, PEMEX, Poza Rica, Veracruz, Mexico. [email protected] ABSTRACT The Altar basin in northwestern Sonora, Mexico, is a subsidiary basin forming a now inactive part of the Colorado River delta. Its sedimentary record illustrates how the delta prograded in the last 4–5 Ma over a late Miocene, structurally distinct, marine basin at the northern end of the Gulf of California. Our interpretation of outcrop data, and data from seven exploratory wells, six analog seismic lines of Petróleos Mexicanos (PEMEX), and magnetic and gravity surveys from various sources indicates the existence of three sedimentary sequences, A, B, and C, which can be correlated at regional scale and have a thickness >5 km at the basin depocenter. The lower sedimentary sequence A is a shale unit representing open marine conditions (outer neritic). It grades into a thick sequence of interstratifi ed mudstone, siltstone, and sandstone (sequence B), which grades in turn into poorly consolidated sand (sequence C). Extensive outcrops of a sandy, cut and fi ll succession exposed along the coast of Sonora are consistent with sequences B and C being the sub-aqueous and the sub-aereal parts of the delta, respectively. A contact at the base of the sequence A, where pre-marine continental deposits are missing, and where the marine sequence overlies crystalline basement, is interpreted as tectonic transport along a top-to-the-northwest detachment fault. The Altar basin became inactive as result of the westward shift in the locus of tectonic activity from the Altar fault to the Cerro Prieto fault, coupled with realignments in the course of the Colorado River during Pleistocene time. Key words: stratigraphy, rifting, Colorado River delta, Altar basin, Gulf of California. RESUMEN El Desierto de Altar en el noroeste de Sonora, contiene una cuenca subsidiaria ubicada en la parte inactiva del delta del Río Colorado. El registro sedimentario ilustra cómo el delta progradó sobre una cuenca marina estructuralmente independiente del Mioceno tardío, hacia la terminación norte del Golfo de California. La interpretación de datos de afl oramientos, datos de siete pozos exploratorios y seis líneas sísmicas analógicas de Petróleos Mexicanos (PEMEX), además de datos magnéticos y gravimétricos compilados de diversas fuentes, indican la existencia de tres secuencias sedimentarias, A, B y C, con más de 5 km de espesor y con una distribución de escala regional. La secuencia inferior (secuencia A) es una unidad de lutita marina de ambiente nerítico externo. Esta unidad grada a una potente secuencia de lodolita, limolita y arenisca (secuencia B), la cual a su vez grada a una secuencia de arena poco consolidada (secuencia C). Las secuencias B y C se interpretan como parte del sistema submarino y subaéreo, respectivamente, que progradó hacia la cuenca marina. Afl oramientos de una secuencia arenosa de corte y relleno, interpretada como depósitos fl uviodeltaicos y expuesta a lo largo 2 Pacheco et al. de la costa de Sonora, son consistentes con una interpretación en donde la secuencia C forma parte de la planicie fl uvial del Río Colorado. El contacto discordante de la base de la secuencia A sobre el basamento cristalino y la ausencia de depósitos premarinos se interpreta como el resultado del transporte tectónico a lo largo de una falla de despegue con transporte de la placa superior hacia el nor-noroeste. La subsidencia en la cuenca de Altar cesó debido al cambio en la localización de la actividad tectónica de la falla Altar a la falla Cerro Prieto, así como al cambio en el cauce del Río Colorado hacia el Valle de Mexicali durante el Pleistoceno. Palabras clave: estratigrafía, rifting, delta del Río Colorado, Cuenca de Altar, Golfo de California. INTRODUCTION lacustrine environments that were periodically fl ooded by discharges of the Colorado River (Dibblee, 1984; Winker The Colorado River delta largely fi lls the depression and Kidwell, 1996). known as the Salton trough, comprising the Imperial Valley, Besides sharing a complex structural control, the in Southern California, and the Mexicali Valley, in northern individual basins also share an early stage dominated by Baja California (Figure 1). The trough is traversed by ac- marine deposits, framed by basin-margin alluvial deposits, tive faults associated with the southern San Andreas fault which predated rapid deltaic progradation into the marine system and the Gulf of California extensional province, basin. However, marine conditions may have persisted on and so the Colorado delta is somewhat unique among the both fl anks of the delta because the rift margins continued world’s major deltas in being structurally confi ned and lying to widen and subside. The Altar basin, east of the active across a major plate boundary between the Pacifi c and North modern delta (Figure 1), is a good example of an abandoned American plates (Figure 1) (Elders, et al., 1972; Lonsdale, basin, whereas the Laguna Salada to the west is a good 1989). The delta also contains a record of the erosion of example of a still active basin (Dorsey and Martín-Barajas, its source in the Grand Canyon and Colorado Plateau in 1999; Martín-Barajas et al., 2001), although both are the Arizona (Winker and Kidwell, 1996, and references therein; result of changes in the locus of tectonic deformation. On Elders et al., 2001; Hunt and Elders, 2001). However, to the Sonoran side of the gulf, less is known about the timing date, no synthesis of the geological structure and history of of major subsidence and the stratigraphic record produced the whole delta had been possible because some areas are by the three major geological processes of transtensional still poorly known. Therefore, we used data from onshore tectonics, proto-gulf marine sedimentation and deltaic wells and seismic refl ection profi les from PEMEX (national progradation. petroleum company of Mexico), and from magnetic and The Altar basin covers an area between San Luis Río gravity surveys, to gain insight into the sedimentation and Colorado and Bahía Adair in northwestern Sonora (Figure structure associated with this transform fault plate margin 2). It is assumed that this area was once part of the delta com- and its relationship to the geological history of the Grand plex, although the surface of the Altar basin is now above Canyon and Colorado River (Elders et al., 2003; Espinosa- the level susceptible to fl ooding by the Colorado River and Cardeña and Elders, 2003). is an extensive eolian dune fi eld. Until now, no stratigraphic The Altar basin in northwestern Sonora shares a data had been published to interpret the nature of the basin common history with other basins within the trough, fi ll, inferred to be at ~5 km depth (Sumner, 1972). and therefore exemplifi es basin evolution in this tectonic Exploratory seismic refl ection surveys and drilling environment. Each of these basins has experienced the ef- for hydrocarbons and geothermal resources have produced fects of three main geological events: 1) an initial period a large amount of data, which for many years remained of extension and subsidence; 2) an intermediate stage of unpublished, apart from a few preliminary reports (PEMEX, marine sedimentation; and 3) a later stage of basin fi lling, 1985; Guzmán, 1981). In this paper, we integrate data from largely dominated by the growth of the Colorado River the Altar basin that include seven well logs and six analog delta, and to a lesser extent by alluvial deposits from the seismic lines (Figure 2). Additionally, we use the gravity basin margin. Where age constraints for these three geologic and magnetic anomalies to interpret the structure of the events exist, the onset and end of this sequence differs from Altar basin. We also studied the sedimentology of outcrops basin to basin, depending on its position relative to major along the western margin of the Altar basin (Figure 2) to depocenters and on the confi guration of the delta complex. gain insight of the sedimentary environments for the up- The construction of the Colorado River delta isolated the permost part of sequence. Then, we contrast our model of Altar basin and the northern gulf from basins located to the the structure and stratigraphy of Altar basin to other co-ge- north and west within the Salton trough. Also, in the north- netic basins located to the west and northwest in the Salton central part of the trough, the growth of the delta produced trough, and fi nally, we discuss the tectonic implications of Stratigraphy and structure of the Altar basin of NW Sonora 3 Salton Sea SAF WSD WSD S2-14 Colorado River 33°00’ Imperial Valley East AL Mesa Gila River IF Yuma California LSF S. Cucapah Mexicali Yuma Basin Baja California Valley Cerro San Luis R.C. Laguna Prieto Salada S. El Mayor Arizona El RosarioSierra Salton CPF Sonora Sea Sierra Juárez ALTAR 32°00’ SMD BASIN USA AF MEXICO El Golfo de Baja CaliforniaWB Sierra Santa Clara Caborca Las Tinajas DB Sonora Gulf TB of Bahía Adair IT Pacific California Ocean MGE Wagner Km Basin 0 20 40 31°00’ 117°00’ 116°00’ 115° 00’ 114° 00’ Figure 1. Digital elevation model (DEM) map, and main tectonic features in the northern Gulf of California and Salton trough regions.