Bol. Depto. Geol. UniSon, 2005, Vols. 18 y 19, p. 153 - 174
OLIGO-MIOCENE EVOLUTION OF SONORA-BAJA CALIFORNIA, MEXICO - A SEGMENT OF THE NORTH AMERICAN MARGIN
Luigi RADELLI Departamento de Geología, Universidad de Sonora, Hermosillo, Sonora, México. email:
[email protected]
ABSTRACT The data obtained during a revision of the Cainozoic stratigraphic sequence of the eastern part of Southern Baja California (SBC) were integrated with previuos knowledge on the Cainozoic stratigraphy of Sonora. In this way it has been possible to recognize the main events that determined the Cainozoic evolution of this segment of the North America margin, prior to the Pliocene (±4.5 Ma) inception of the San Andreas Transform System. During the Eocene a clastic sedimentation took place in southern California (Poway Conglomerate) and in Southern Baja California (Tepetate Formation of conglomerate and sandstone). Toward the uppermost Eocene/lowermost Oligocene, upon the uplift of the Huepac Eocene Volcanic Arc, and of the Eocene Mountain Chain of Sonora (see Radelli and Navarro, this volume), the basin of SMO took origin eastwards. In Sonora, during the Oligocene (±35 to ±25 Ma) the Novosaigame Conglomerate was covered by the famous rhyolitic-ignimbritic volcanic pile of Sierra Madre Occidental (SMOc). At about 25 Ma an extensional phase intervened: within the western part of SMO the volcanism became bimodal, whereas still new basins, the so-called "Belleza" and Magdalena" basins, were opened westwards. These latter were filled up by lacustrine, detrital, sometimes boratiferous, deposits and interlayered flows of basalts, that in places show a trachyandesitic character. At ±20 Ma a shortlived pulse of fracturing allowed the empla-cement of a number of huge rhyolitic dykes. It was followed by one of the main extensional phases, the Baucarit extensional phase. Then, a number of intramontane, fluviatile basins were opened in western Sonora. There, unconformably upon all of the pre-existing rocks, were accumulated, prior to 12.5 Ma, conglomerates, sandstones and flows of basaltic andesites, collectively termed Baucarit Formation. In Western Sonora new deposits began to be accumulated upon a Mesozoic basement only during the early Miocene : Gipo Baleado Tuff (22.5 Ma), unconformably followed by the LaEsperanza Andesite (22.7 Ma). Also in this block the ±20 Ma old rhyolitic dykes occur (Rancho El Gavilan).From 18.8 to 15.3 Ma, whereas the Baucarit Formation is depsited eastwards, in this western sonoran block the El Mezquite Dacite is accumulated, which is intruded by a granophyre. And between 15 and 12.5 Ma both the El Mezquite Dacite and the granophyre intrusive into it were unconformably covered by La Pasion Andesite.
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In the eastern part of Southern Baja California during the Oligocene-middle Miocene time span (until ±12. 5 Ma) a thick (> 1000 m) detrital continental sequence (sandstone and conglomerate) with interlayered andesitic and basaltic flows, aglomerates, and tuff, was deposited on its eastern part. This sequence is comprised of El Salto and Pelones Formations in the area of Concepción Bay, and of Las Parras Formation in the area of Loreto and Sierra de la Giganta. It appears to grade westwards into the marine formations of San Gregorio and San Isidro of the Southern Baja's Pacific coast. The unconformities of this time-span noted in Sonora do not occur in Southern Baja. A link between these formations and the coeval ones of Sonora is established, however, by interlayered or cross-cutting magmatic products (for instance: 28 Ma tuff of El Salto Formation, 17 to 15 Ma andesitic dykes cutting the lower part of the Las Parras Formation, 13.3 Ma basaltic flows interbedded in the upper part of this same formation). Also, to the ±20 Ma rhyolitic dykes of Sonora correspond in SBC coeval tonalitic intrusions into the Pelones Fomation and dacitic intrusions into the Las Parras Formation. At 12.5 Ma the subduction zone, active until then at the western limit of Baja, ceased to exist and its place was taken by the Tosco-Abreojos Transform Fault. This lithospheric reorganization brought about a new extensional phase all over the area. In Sonora, the Baucarit deposits were tilted and rotated by listric faults. In western Sonora, they were subsequently unconformably covered by the ±12-10 Ma acidic volcanics, the Lista Blanca Volcanics. In SBC the Las Parras Formation was affected by a regional extensional cleavage, and, at least locally, gently tilted. Then, between ±12 and ±10 Ma, the Las Parras Formation of Sierra de la Giganta and, in the southern part of the Concepción Peninsula, the Pelones Formation were unconformably, or at least disconformably, covered by the clastic deposits with a tuffaceous matrix of the Comondú and Hornillas Formations, respectively. Further eastwards, beginning with the Minitas Formation of the northern part of the Concepción Peninsula, a conspicuous gypsiferous belt appears at El Carmen Island and extends itself northwards to Mulegé, Santa Rosalia and as far as Laguna del Gato (a total distance of some 250 Km). This gypsiferous structure, the main result of the aforementioned extension, can be interpreted as an aborted rift, or at least as a Protogulf of California. In western Sonora the La Ventana Formation was unconformably covered by the tuff of the Sanat Ursula Formation (12.3 to 11.4 Ma). In eastern Southern Baja California a short-lived acidic volcanic activity occurred in the area of Santa Rosalia (copper bearing Boleo Formation) and, westwards, in that of San Ignacio - Sierra San Francisco. A new extensional phase intervened at ±10 Ma, which both in geological age and tectonic style corresponds to the classic Basin and Range extension of the western USA. In western Sonora, the ±12-10 Ma Lista Blanca Volcanics, together with the upper part of their basement, were rotated and attenuated by a set of ±SW-dipping listric faults. Such crustal attenuation was strong enough to give way to an elongated marine embayment, which is known as the Paleogulf of California. In western Sonora, the afore mentioned tilted rocks were unconformably covered by still flat-lying ±86 Ma old basalts. The ±10 Ma old listric faults of Sonora do not occur at surface in SBC, meaning that before reaching it they had already entered a ductile zone. As a consequence the Paleogulf of California was an asymmetrical structure, which is still revealed by the morphology and the bathymetry of the continental crust on either side of the Gulf of California. Nevertheless, that deepseated extensional conditions existed then in Southern Baja is clearly suggested by the huge blanket
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of almost flat-lying basalts and trachyandesitic basalts (Comondú Trachyandesitic Basalt, Ricason Formation, Pilares Formation, Esperanxa basalt) erupted there between ±10 and ±6 Ma. As a whole, the structural pat-tern under discussion suggests the prelude of a rift. It was superseded anyway by the Pliocene (±4.5 Ma) intervention of the San Andreas Transform System, that opened the Gulf of California, and caused Sonora and Baja to belong in two different plates. RESUMEN Los datos obtenidos en una revisión de la secuencia estratigráfica cainozoica de la parte oriental de Baja California Sur han sido integrados con conocimientos previos de la estratigrafía cainozoica de Sonora. De esta manera ha sido posible reconocer los principales eventos que determinaron la evolución de este segmento de la margen de Norte América durante el Oligoceno y el Mioceno, es decir antes de la intervención del Sistema Transformante de San Andrés a los 4.5 Ma (Plioceno). Durante el Eoceno una sedimentación clástica tuvo lugar en la parte occidental del Sur de California (Conglomerado Poway) y de Baja California Sur (conglomerados y areniscas de la Formación Tepetate). Hacia el final del Eoceno/comienzo del Oligoceno, en relación con el levantamiento del Arco Volcánico Huepac del Eoceno, y de la Cadena de Montañas del Eoceno de Sonora (veáse Radelli and Navarro, este volumen), tomó origen al Este la cuenca de la Sierra Madre occidental (SMOc), donde se depositó en Conglomerado Novosaigame. Durante el Oligoceno (±35 a ±25 Ma), en Sonora oriental el Conglomerado Novosaigame fue cubierto por la famosa secuencia riolítico-ignimbrítica de la SMOc. Alrededor de los 25 Ma intervino una fase distensiva : en la parte occidental de la SMOc el volcanismo se volvió bimodal, mientras que nuevas cuencas – las así llamadas Cuencas « Tubutama », « Magdalena » y « Belleza » se abrieron más al Oeste. Estas cuencas se rellenaron de depósitos lacustres, detríticos a veces boratíferos con intercalaciones de flujos basálticos, que a veces presentan un caracter traquiandesítico. A ± 20 Ma una corta fase de fracturación permitió el emplazamiento de numerosos grandes diques riolíticos. Dicha fase fue seguida por una de las mayores fase distensivas, la fase Báucarit. Varias cuencas fluviales intramontanas se abrieron entonces en Sonora occidental. Entre ±20 y 12.5 Ma se acumularon en esas cuencas, y discordantes sobre todas las rocas pre-existentes, conglomerados, areniscas con flujos intercalados de andesitas basálticas, depósitos conocidos bajo el nombre de Formación Báucarit. En Sonora Occidental los depósitos empezaron solamente en el Miocene inferior: Tobas Gipo Baleado (22,7 Ma), seguida en discordancia por la Andesita La Esperanza (22.7 Ma). En este bloque occidental de Sonora también se encuentran los diques riolíticos a los cuales se asigna una edad radiométrica de ±20 Ma. Entre los 18.8 y los 15.3 Ma, cuando al Este se acumula la Formación Báucarit, en el bloque occidental ocurre la Dacita El Mezquite, misma que es afectada por la intrusión de un granofiro. Entre 15 y 12.5 Ma tanto la Dacita El Mezquite como la intrusión granofírica estám cubiertas en discordancia por la Andesita La Pasión. En la parte oriental de Baja California Sur durante el Oligoceno y Mioceno inferior y medio (hasta ±12.5 Ma) se depositó una espesa (> 1000 m) secuencia continental de areniscas y conglomerados con intercalaciones de flujos andesíticos y basálticos, aglomerados y tobas. Esta secuencia
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incluye las Formaciones El Salto y Pelones del área de Bahía Concepción, y la Formación Las Parras en el área de Loreto y de Sierra de la Giganta. Hacia el Oeste esta secuencia pasa progresivamente a las Formaciones marinas de San Gregorio y San Isidro de la costa pacífica de la Baja California Sur. Las discordancias que en el periodo considerado ocurrieron en Sonora no existen en Baja Califoria Sur. Un enlace entre las formaciones citadas de Baja California Sur y las de Sonora se establece sin embargo gracias a las rocas ígneas que se intercalan en ellas o las cortan (por ejemplo : toba fechada 28 Ma de la Formación El Salto; intrusiones de ±20 Ma, tonalíticas en la Formación Pelones y dacíticas en la Formación Las Parras, que corresponden a los diques riolíticos de la misma edad radiométrica de Sonora ; diques andesíticos de 17 y 15 Ma que cortan la parte inferior de la Forma-ción Las Parras, y flujos basálticos de 13.3 Ma intercalados en la parte superior de esa misma formación. A los 12.5 Ma la zona de subducción activa hasta entonces al límite occidental de Baja dejó de existir, su lugar siendo tomado por la Transformante Tosco-Abreojos. A esta reorganizción litosférica siguió en toda la región considerada una nueva fase distensiva. En Sonora los depósitos Báucarit fueron basculados y rotados por fallas lístricas. Tanto los depósitos Báucarit como la Andesita La Pasión fueron entonces cubiertos discordantemente, entre ±12 y ±10 Ma, por las rocas volcánicas ácidas de la Formación Lista Blanca y de la Formación La Ventana (12.3–11.4 Ma), respectivamente. Contem-poráneamente, en Baja California Sur la Formación Las Parras fue afectada en su conjunto por un clivaje distensivo y localmente ligeramente inclinada. Y entre ±12 y ±10 Ma la Formación Las Parras de la Sierra de la Giganta y la Formación Pelones de la parte sur de la Peninsula Concepción fueron cubiertas en discordancia angular, o por lo menos en disconformidad, por los depósitos clásticos con una matriz tobácea de la Formaciones Comondú y Hornillas respectivamente. Más al Este, empezando con la Formación Minitas de la parte norte de la Peninsula Concepción, aparece un conspicuo cinturón yesífero, mismo que se desarrolla desde la Isla El Carmen hasta la Laguna del Gato, pasando por Mulegé, la Isla San Marcos y Santa Rosalía, es decir sobre una distancia de unos 250 km. Esta estructura yesífera es el resultado principal de la ya mencionada distensión y puede ser interpretada como un rift abortado, un Protogolfo de California. En Sonora occidental la Formación La Ventana fue entonces cubierta discordantemente por las tobas de la Formación Santa Ursula (12.3 a 11.4 Ma). Mientras, en la parte oriental de la Baja California Sur una corta fase de actividad volcánica ácida tuvo lugar en las áreas de Santa Rosalía (Formación cuprífera El Boleo) y de San Ignacio-Sierra de San Francisco más al Oeste. Una nueva fase distensiva intervino a los ±10 Ma, la cual tanto por su estilo como por su edad corresponde a la clásica distensión Basin and Range del Oeste de los Estados Unidos de América. En Sonora occidental las rocas volcánica de la Formación Lista Blanca (±12 a 10 Ma), junto con la parte superior de su basamento estratigráfico. Fueron rotadas y atenuadas por un conjunto de fallas lístricas con echado al SW. Esta atenuación cortical fue lo bastante importante por dar lugar a una cuenca marina alargada, conocida como Paleogolfo de California. Las formaciones de Sonora occidental mencionadas arriba fueron entonces cubiertas discordantemente por basaltos todavía horizontales de ±8 a 6 Ma. Las fallas lístricas de ±10 Ma que afloran en Sonora no ocurren en la superficie en Baja California Sur, lo que sugiere que antes de alcanzarla habían llegado a la zona dúctil. Como consecuencia de este hecho, la de Paleogolfo de California era una estructura
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asimétrica, todavía mostrada por la morfología y la batimetría de los dos lados del Golfo de California. Sin embargo, la existencia en ese tiempo de condiciones distensivas profundas en Baja California Sur está claramente indicada por una espesa cubieta de basaltos y traquiandesitas basálticas (basaltos traquiandesíticos Comondú, Formaión Ricasón, Formacón Pilares. basalto Esperanza) emplazados entre ±10 y ±6 Ma. El modelo estructural resultante sugiere el preludio de un rift. Este sin embargo ha sido suplantado por la intervención del Sistema Transformante de San Andrés que abrió el Golfo de California e hizo que Sonora y Baja pertenezcan a dos placas diferentes.
INTRODUCTION Prior to the Pliocene (± 4.5 Ma) inception of the San Andreas Transform System, and the consequent opening of the Gulf of California (fig. 1), Sonora and Baja California were two adjacent strips of a single structure, the Cainozoic North American Margin, bounded, until 12.5 Ma (Klitgord and Mammerickx, 1982 ; Mammerickx, J. and Klitgord, K.D., 1982) by a subduction zone. However, these two strips have seldom been considered together in order to clarify the Cainozoic evolution of said margin as a whole. And when this has been done, it has been done taking into account, essentially, its volcanic products (e.g., Gastil et al., 1979; Radelli et al., 1993), thus mostly disregarding its sedimen-tary formations. There are two reasons for this. The first one is that most of the geologists who worked in Baja California did not know Sonora, and vice versa. The second one is an unruly employment in Baja California, and particularly along its western side, of the litho-stratigraphic terms (formations), which precluded all stratigraphic and tectonic
reasoning and interpretation. For instance, the term of Co-mondú Formation was introduced by Heim (1922) for a Upper Miocene sequence beginning
Fig. 1 – General outline of Sonora, Gulf of California and Baja California.
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with clastic rocks and containing at its top ±10-6 Ma old volcanics of the Comondú area. But, later on , this same term has been applied to a number of clastic and volcanic occurrences of western Baja, irrespectfully of their stratigraphic position and geological age (Radelli and Ortiz, 1995). As a consequence, whereas the Comondú Formation of Heim (1922) is capping the Sierra de la Giganta, according to the post-Heim (1922) geological literature, a supposed “Comondú Formation” would constitute the base of the Cainozoic stratigraphic pile of Western Southern Baja California, in particular at Loreto, Bahia Concepción and Santa Rosalia, where OligoceneLower Miocene both sedimentary and igneous rocks actually occur. It is the purpose of this paper to try and fill up this gap. What follows is based on previous knowledge of the Cainozoic stratigraphy and tectonics of Sonora, acquired by the author during some 20 years of work there; on a field revision of the Cainozoic stratigraphy of Southern Baja California carried out, from 1995 to 1998, by the author and one of his students (Ortiz, 1998); and, finally, on a chro-nostratigraphic correlation between the Caino-zoic formations of both areas. STRATIGRAPHY AND TECTONICS EOCENE
The stratigraphy and tectonics of the Eocene has been dealt with in a companion paper (Radelli and Navarro-H., this volume), so that they will be only summarized here. During the Early Eocene (±50 to ±40 Ma), western Sonora and Baja California – the
Baja–Bor-derland terrane of Abott and Smith (1989) – were located some 1500 km southwestwards of their present position, so that San Diego was oughly at the latitude of Mazatlan. During that time span (±50 to ±40 Ma) due to a magma underplating, a huge Conti-nental Volcanic Arc, the Huepac Arc, which included both intrusive and effusive products, was built up in what is now axial Sonora (Radelli et al., 1992; Lucero and Radelli, 1993; Radelli et al., 1995; Radelli and Lucero, 1996a; Radelli and Lucero, 1996b; Radelli, 1999). During the late Eocene (±40 to ±35 Ma) the Baja-Borderland terrane of Abbott and Smith (or Baja-Borderland Block) was moved north-eastwards and rotated some 30º clockwise (Fig. 2). Then the Huepac Arc was affected by a strong compressional phase (Bronner and Radelli, 1996), and uplifted. Upon its uplift such arc became the structural backbone of Northwest Mexico, bounding the basin of the Sierra Madre Occidental
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Huepac Arc ; 4 : Oligocene basin of the SMOc (SMOc) on the east [It is worth noting that, in order to explain the clastic sediments of Baja (see in particular their fig. 2 A), Karig and Jensky (1972) had already postulated such an arc. However, for wanting of valid stratigraphic and chronological data they postulated its existence at a wrong date (25 Ma).] During latemost Eocene, the erosion of the Eocene Arc furnished detritus to the Novosaigame Conglo-merate with which began the Cainozoic history of the basin of the SMOc (Bockoven, 1980); Cochemé, 1985; Cochemé and Demant, 1991). During the Eocene, the western side of Southern Baja California was characterised by the deposit of the chiefly conglomeratic Tepetate Formation. OLIGOCENE to EARLY MIOCENE (± 20 Ma) OF SONORA
Fig. 2 – A outline of the Baja-Borderland Block and of the Eocene Huepac Arc. Baja California is shown in its position prior to the opening of the Gulf of California. 1 : Baja-Borderland Block and 2 : its Eocene deposits ; 3 : Early Eocene
In eastern Sonora, during the Oligoceneearly Miocene (±35 to ±20 Ma) the Novosaigame Conglomerate was covered by the volcanic pile of the Sierra Madre Occidental. This pile consists, upwards, and above some andesiditic flows, of the huge rhyolitic-ignimbritic sequence that actually characterizes that Sierra, and, beginning at about 25 Ma, of a bi-modal volcanic sequence of alternating rhyolite and andesitic basalt (Cochemé, 1985; Cochemé and Demant, 1991). It should be noted that the rhyoliticignimbritic activity is divided from the bimodal volcanism by an intervening deformational phase. Such phase is revealed by a number of syncline-like structures that affect the rhyolites-ignimbrites exclusively (Radelli,
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1986). The bimodal sequence is accompanied in place by sedimentary clastic deposits, often misinter-preted in the past as bilonging to the Baucarit Formation (see below). In the axial part of Sonora (fig. 3), to the west of the Sierra Madre Occidental proper (Baviacora, La Estrella, Mazatan, MagdalenaTubutama), a concomitant extensional phase occurred as well at about 25 Ma. It gave way to the exhumation of the Sonora Metamorphic Core Complexes of Magdalena and Mazatan and to the opening of several lacustrine basins, generally called « Magdalena », « Tubutama », and « Belleza » basins.
flows (Miranda et al, 1998; Calles, 1999). The above mentioned syncline-like structure of the SMOc that con-tain the bimodal volcanic rocks and the clastic deposits that accompany them appear as correlative with the « Mgdalena », « Tubutama » and « Belleza » basins of axial Sonora. It is possible that other basins of this type existed also in western Sonora, as for instance in the Sierra of Santa Ursula, near Guaymas, where 23.5 Ma old volcanic rocks of intermediate composition (Grijalva and Roldan, 1998) occur below the 12-10 Ma Lista Blanca Volcanics (see below). In western Sonora (Mora-Alvarez and McDowell, 2000) the first Cainozoic deposit resting upon the Paleo-( ?)-Mesozoic basement is that of the ignimbritic Gipo Baleado Formation (23.5 Ma). This formation is unconformably followed by the La Esperanza Andesite Formation (22.7 Ma) , which consists of andesite and interlayered sandstone hori-zons. Thus, prior to ±20 Ma (see below) western Sonora still preserves its individuality from the rest of Sonora as it had done during the Eocene, when it was divided from it by a transcurrent fault (Radelli and Navarro-H., this volume) OLIGOCENE to MIDDLE MIOCENE (±35 to ±20 Ma) OF SOUTHERN BAJA
Fig. 3 – Map showing the localities of Sonora mentioned in the text.
During the Oligocene-middle Miocene (until 12.5 Ma) interval, a detrital sedimentation occurred in eastern Southern Baja (Figs. 2 and 4).
Concomitantly with the bimodal volcanism of the SMOc, these basins were filled up by clastic and sometimes boratiferous deposits, and by interlayered sub-alkaline basaltic (s.l.)
In the area of Concepción Bay (McFall, 1968) this sedimentary pile includes two
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formations: the El Salto Formation, and the Pelones Formation (Fig. 5). The El Salto Formation unconformably covers a pre-Cainozoic basement, including some Cretaceous (?) intrusive body. It consists of mostly quartzitic, fine to middle grained, poorly sorted and poorly rounded, cross bedded red-dish sandstones and conglomerates, with inter-bedded tuff radiometrically dated (K/Ar) 28 Ma (late Oligocene). The Pelones Formation rests conformably upon the El Salto Formation. It consists of a huge pile (> 1000 m) of volcaniclastic deposits (mostly andesitic agglomerates and conglome- rates), with interbededd basaltic flows and tuffaceous sandstone, intruded by ± 20 Ma old tonalitic bodies (see below).
Fig. 4 – Map showing the localities and the geographic units of Southern Baja California mentioned in the text.
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In the area of Loreto and Sierra de la Giganta, the morphologic backbone of Southern Baja, the equivalent of the El Salto and the Pelones Formations is the also detrital, about 1000 m thick, sub-horizontal to gently tilted, Las Parras Formation (Ortiz, 1998). The lowermost part of Las Parras Formation, possibly in contact with the preCainozoic basement, does not crop out in the here considered area. Its exposed section begins with cross-bedded, fine grained deposits (shales) interfingering with conglomerate, sandstones containing lenticular bodies of microconglomerate with volcanic fragments, and some interbedded basaltic flows. Upwards, the Las Parras Formation becomes chiefly conglomeratic, and includes interbedded andesitic and basaltic flows as young as 13.3 Ma (Gastil et al., 1979). The matrix of the conglomerates is generally sandy, sometimes tuffaceous. Its colour varies from brown to orange, and to red. The clasts of the conglomerates are of volcanic rocks, including andesite, dacite, basalts, and tuff. In the eastern part of Sierra de la Giganta they are poorly sorted and gently rounded, and vary in size from some 5 cm to about 70 cm; but the deposits of this formation become much less coarse towards the west. The Las Parras Formation has been intruded first by ±20 Ma old dacitic bodies (see below), and then, in its lower part, by from 5 to 20 m thick andesitic dykes, radiometrically (K/Ar) dated 16±2.0 and 15.4±0.8 Ma (Gastil et al. 1979). The Las Parras Formation is affected by a strong, regional, extensional vertical cleavage related to a normal faulting , a character that is not shown by the overlying, younger formations (e.g., the Comondú Formation - see below).
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The age of the upper limit of the Las Parras and its equivalent formations is not perfectly constrained. However, taking into account (i) the 13.3 Ma radiometric age of some of its basalt on the one hand, and (ii) the uncon-formity that divides it from the overlying Comondú Formation, it is permissible to relate such limit, as well as the regional clevage of the Las Parras Formation with the 12.5 Ma old ceasing of the subduction at the western limit of Norhtwest Mexico (Klitgord and Mammerickx, 1982; Mammerickx and Klitgord, 1982), it can be assigned to the Oligocenemiddle Miocene (
