STRATIGRAPHY AND STRUCTURE OF THE PALEN FORMATION, PALEN MOUNTAINS, SOUTHEASTERN CALIFORNIA Richard A. LeVeque Union Oil Company 2323 Knoll Drive Ventura, California 93006 ABSTRACT The Palen Formation is known to occur only in the Palen Mountains of southeastern California The Palen Formation is comprised of three (Fi g. 1). It has been intruded by a porphyriti c members: lithofeldspathic arenite, polymictic rhyodacite dated by K-Ar methods at 175 m.y.b.p. conglomerate and quartzose arenite, deposited and by quartz monzonites dated at 66 m.y.b.p. prior to the development of at least a portion of (K-Ar; Pelka, 1973). The Palen Mountains are a Jurassic magmatic arc that existed in south­ about 43 km northwest of Blythe, California and are eastern California and western Arizona. The com­ separated from the Granite Mountains to the north position of the three members of the Palen Forma­ by Palen Pass. Access to the study area is by tion defines a distinct feldspar to quartz-rich California State Route 177 (Rice Road) north from trend upward in the section. Gradational contacts U.S. Interstate 10 approximately 25.6 km (16 mi.) between each member imply that sedimentation was and then east via an unimproved road to the Palen not interrupted and that drastic changes in sedi­ Pass area. The U.S. Geological Survey Palen mentary environments did not occur. Bedding Mountains 15' quadrangle encompasses the study area. characteristics suggest that the depositional environments record a transition from subaqueous STRATIGRAPHY to subaerial deposition in a basin that shoaled through time. The Palen Formation was first studied and named by Pelka (1973). He chose to divide the Intense deformation of the Palen Formation formation into three members based on litholigic produced thrust and strike-slip faults, tight to characteristics. In ascending stratigraphic isoclinal south-vergent folds and a penetrative order. the three members are: 1) lithofeldspathic cleavage. This deformation occurred during the arenite, 2) polymictic conglomerate, and 3) southward movement of upper plate Paleozoic and feldspathic to quartzose arenite (fig. 2; classifi­ possible Mesozoic metasedimentary rocks along the cation based on Crook, 1960). The base of the Palen Pass thrust over a lower plate consisting of section is not exposed and the uppermost member Palen Formation and an intrusive rhyodacite has been intruded (see Fi g. 3). The absence porphry. Fold axes in the lower plate strata of a complete section combined with the degree of trend roughly E-W, plunge gently to the west­ deformation precludes an accurate estimate of the northwest and are parallel to the Palen Pass thrust thickness of the formation. fault. The similarity of structural fabric data from the upper and lower plate rocks with deformed Member Descriptions rocks to the east in the Big Maria Mountains suggests that this ~eformation occurred in the Lithofeldspathic Arenite early Late Cretaceous (90-100 m.y.b.p.). The lowermost member is gray-green to olive­ INTRODUCT ION green in color and comprised of framework grains of angular plagioclase feldspar embedded in a matrix The Mesozoic sedimentary history of south­ of epidote, green biotite and chlorite. The eastern California is obscure largely because of percentage of matrix ranges from approximately 20% the scarcity of rocks of this age and type. to almost 70%. In a number of samples lithic Regional correlations of sedimentary strata of fragments of volcanic and quartzitic rocks are known Mesozoic age are difficult to make due to present. although small in size and relatively the disruption of sedimentary environments by low in abundance (20% maximum). igneous activity that occurred both during and after sedimentation. The bedding characteristics of the lowermost member are not prominently displayed. Most of Pre-Jurassic, post-Paleozoic sedimentary the exposures of this member are massive and rocks are exposed at scattered localities in the difficult to interpret. Where observed, bedding eastern and southern Mojave desert. These strata is represented by thin, wavy laminations that provide the only paleogeographic details of the range from 1-3 mm to 1-1.5 cm in thickness. This transition in regional geological environments appears to be the most common style of bedding from cratonal sedimentation in the Paleozoic to although both low-angle crossbeds and rare graded arc magmatism in the Jurassic. The Palen Formation beds occur. Texturally this member is poorly is one such sequence of Early Jurassic metamor­ sorted and contains angular to subangular framework phosed sandstone and conglomerate that outcrop in grains. southeastern California. This paper describes their occurrence, stratigraphy and structure and These observations (abundant plagioclase, presents speculative paleogeographic interpre­ lithic fragments, poor sorting and massive bedding) tations based on this data. suggest that this member was rapidly deposited 267 115 34 cr .....O' o C> 33 ARIZ ._.-."""'(. MEX "'-. Figure 1 - Location map of study area. A = Arica Mountains; BM = Big Maria Mountains; CH = Chuckwalla Mountains; C = Coxcomb Mountains; D = Dome Rock Mountains; E = Eagle Mountains; G = Granite Mountains; H = Hexie Mountains; LM - Little Maria Mountains; MC = McCoy Mountains; M= Mule Mountains; P = Palen Mountains; PP = Palen Pass; PI = Pinto Mountains; R = Riverside Mountains. with only minor reworking. The sedimentary charac­ interbedded with thin, discontinuous beds of feld­ teristics of this member are similar to the pelitic­ spathic arenite. Relative percentages of the two arenaceous facies II of Mutti and Ricci-Luchi's lithologic types are 75% conglomerate. and 25% model of turbidite sedimentation (1978 English arenite. translation of the original 1972 Italian paper by T. Nilsen). In their model sediments of this Quartzite is the dominant clast type (range facies are deposited on the outer fan apron where 40-100%), followed by leucogranite (0-50%), channeling is of less importance for the distri­ carbonate (0-30%) and minor resedimented cobbles bution of turbidity flows. of the lower member of the Palen Formation. Volcanic clasts are extremely rare within this Polymictic Conglomerate member. Most of the clasts are well rounded although angular clasts do occur throughout the The middle member of the Palen Formation is section. The most pronounced aspect of this comprised of polymictic conglomerate that is member relative to the lowermost member is the 268 Q QUARTZOSE ARENITES A--\90-./\.. SUB ABILE ARENITES () () ~- x. a.<::(' S Cl)UJ 'w QI- ~t: -12 ffUJ ~m ICC Q..~ o <::(' (/) 0 ~ ..J uJ -I- u.. FHDSPATHIC LABILE ARENITES F R 75 50 25 Figure 2 - Generalized QFR diagram for arenites; from Crook, 1960. increase in the abundance of quartz, both as part of this member are massively bedded. Actual separate detrital grains and as polycrystalline bedding traces in the lower part of this member may quartzite fragments. be obscurred by the strongly developed cleavage. The middle(?) to upper parts of this member are It is apparent that these sediments underwent dramatically trough cross-bedded on a very large deposition with minor consequent sorting or redis­ scale. Single sets of some cross-strata are at tribution into graded sequences. Deposition in an least 10 Min height. These structures are best alluvial-fan system or within the inner-fan of a exposed on the east side of the Palen Mountains, subsea canyon are plausible alternatives for the topographically below the ridge of rhyodacite environment of deposition of this member. An porphry that is intruded into this uppermost inner-fan environment is favored because of its member. The morphology of these sets (tabular, logical succession to the inferred outer fan wedge-shaped) is difficult to determine because environment described for the first member. It most of these beds are vertical and exposed along may be that although the conglomerates in the a steep slope. lower part of this member were deposited in a subaqueous environment, deposition of the conglo­ The transitional contact between the uppermost merates in the upper part of the member (and up and middle members of the Palen Formation suggests into the transition zone between this and the that a significant hiatus in deposition did not upper member) occurred in a subaerial alluvial fan occur between them. A complex mixed environment environment. Identification of those 'parts that incorporated waning-stage coastal alluvial deposited subaqueously cannot be made in the fan sedimentation with encroaching eolian sand absence of marine fossils. Such fans are des­ deposition may have existed during the deposition cribed by Rust (1980) as "coastal alluvial fans". of the uppermost middle member and the lowermost upper member. This interpretation is based upon Quartzose Arenite the assumption that depositional environments were logically successive. The uppermost member of the Palen Formation ranges from subarkose to quartzose arenite in Age of the Palen Formation composition. The greatest amount of feldspar is found above and within the transition zone between The intrusive contact with the rhyodacite the middle and upper members. Original textural porphry provides the only direct constraint on the details of the quartz grains have been obliterated age of the Palen Formation. Although an isotopic by deformation. An examination by luminoscope of age of 175 m.y.b.p. for the rhyodacite was derived three samples entirely composed of quartz revealed by K-Ar analysis (Pelka, 1973),