BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l . 39 . pp. 1087-1102 December 30.'1928 SIGNIFICANCE OF THE MATILIJA OVERTURN1 BY PAUL P. KERR AND HUBERT G. SCHENCK (Read before the Society December SO, 1927) CONTENTS Page Introduction................................................................................................................ 1087 Strata at Matilija .................................................................................................. 1089 Order and thickness .................................................................................... 1089 Chico form ation............................................................................................... 1089 Tejon form ation................................................................................................ 1089 Sespe formation ............................................................................................. 1091 The Matilija overturn ............................................................................................. 1091 Significance of the overturn ................................................................................. 1096 Conclusions ............................................................................................................... 1101 I ntroduction 2 The Matilija overturn is on the southern margin of the Santa Ynez Mountains, 30 miles east of Santa Barbara and 80 miles northwest of Los Angeles, in Yentura County, California. The name is taken from a station on the Ventura-Ojai Branch of the Southern Pacific Railroad and from Matilija Springs, on Matilija Creek, 3 miles north of the station. The area in which the principles illustrated by the overturn may be considered significant embraces the coastal district inland from the Santa Barbara Channel. It is a somewhat irregular area beginning 20 miles or more northwest of Los Angeles and extending northwestward to Point Conception (figure 1). The conclusions given in this paper are based on detailed maps prepared during parts of the years 1926, 1927, and 1928 and on several published reports on parts of the area. The structural characteristics of the region near Matilija may be 1 Manuscript received by the Secretary of the Society April 2, 1928. 2 The authors wish to express their appreciation of discussion and constructive criti­ cism offered in the preparation of this paper by Profs. C. F. Tolman, Jr., and Eliot Blackwelder, of Stanford University; Prof. Douglas W. Johnson, of Columbia University; Dr. W. S. W. Kew, Los Angeles, California, and Dr. Thomas L. Bailey, Ventura, Cali­ fornia. (1087) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/39/4/1087/3414991/BUL39_4-1087.pdf by guest on 28 September 2021 on 28 September 2021 by guest f Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/39/4/1087/3414991/BUL39_4-1087.pdf SYMBOLS Structurai axis with uncertain Location Normal anticline Normal syncline 1088 Overturned anticline Overturned oyncline /formai dip Overturned dip Shear zone F ouit KERR AND SC HE N C R THE ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- M ATILIJA O V E R T U R N Thrust fault Pt. Conception CmmeL F ig u r e 1.—Bend in the San Andreas Fault and the General Trend of parallel Structures between it and the Coast The Matilija overturn is shown in the central part of the map. INTRODUCTION 1081) summarized as follows: first, the structural features have been formed by the movement of overthrust blocks from the inland toward the coast; second, the usual features produced by this movement are thrust-faults or overturned folds or combinations of the two; third, the direction of release of pressure is commonly the same, whether the resultant structure is a fold or a fault, the local conditions or the nature of the strata being the deciding elements. The structural feature to which particular attention is here directed extends axially from east to west and is a broad anticlinal fold of massive beds of sandstone and shale, which are overturned toward the south. The fold offers little novelty, considered as a structural type, for an asymmetrical arch of this kind is to be expected in massive and mod­ erately competent strata that have been compressed under a com­ paratively light load. The feature is worthy of attention, however, in view of the significant relation it bears to the general deformation in the southern Coast Ranges of California. S t r a t a a t M a t i l i j a ORDER AND THICKNESS Three sedimentary formations are exposed at Matilija. The oldest is the Chico (Cretaceous), the youngest is the Sespe (Oligocene), and be­ tween the two is the Tejon (Eocene). The total thickness of these formations is about 16,500 feet, divided about as follows: Feet Sespe (Oligocene (?)) .................................................................... 4,000 Tejon (Eocene) ................................................................................ 7,500 Chico (Cretaceous) .......................................................................... 5.000 * CHICO FORMATION Gray to black indurated shale, with intercalated beds of arkose sand­ stone, predominates throughout the Chico formation. Ripple-marks, mud-cracks, cross-beddijig, ellipsoidal nodules, concretions, fracture cleavage, and slickensides are common, but fossils are scarce. Those that have been collected include poorly preserved ammonites, Inoceramus, and a few species of gastropods and pelecypods, as well as numerous remains of plants and some arenaceous Foraminifera. The base of the formation is not exposed in the immediate vicinity of the Matilija over­ turn. TEJON FORMATION The rocks of Eocene age are divided into three lithologic units, each of which carries marine fossils that are regarded as characteristic of the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/39/4/1087/3414991/BUL39_4-1087.pdf by guest on 28 September 2021 1 0 9 0 KERR AND SCHENCK----THE MATILIJA OVERTURN Tejon formation of California. The necessity for giving these units specific names has become apparent, and two new names are therefore suggested—Matilija sandstone and Cozy Dell shale. The youngest unit of the Tejon formation is the Coldwater sandstone, already named and defined by Watts.3 The Matilija sandstone rests without apparent angular discordance on the Chico shale. The massive arkose at the base of the Matilija is more resistant than the underlying shale and forms a convenient guide for differentiating the formations. This unit, comprising beds about 2,500 feet thick, in which sandstone predominates over shale, is well exposed in the canyon at Matilija Springs,4 selected as the type locality, as well as on the top of Topatopa Bluff, on the south side of Santa Paula Ridge, and on San Cayetano Mountain. Near the springs is a bed of lignitic facies, distinguished by abundant specimens of Metacerithium, Ostrea, and other mollusks embedded in a green and purplish sandy shale, and the same bed is found farther east, at the same position in the geologic column. Other fossils collected from scattered localities include Nekewis io (Gabb), Tturritella wvasana Conrad, Meretrix hornii (Gabb), Pitanria uvasana (Conrad), Glycymeris sagittata (Gabb), Psammobia hornii (Gabb), and Spatangus tapinus Schenck. The Cozy Dell shale lies disconformably above the Matilija sandstone and below the Coldwater sandstone. The name Cozy Dell is proposed for the rhythmically bedded green micaceous shale and sandstone that is typically exposed in Cozy Dell Canyon, on the east side of Ventura River. This unit has an aggregate thickness of about 2,500 feet and carries such molluscan fossils as Amaurellina moragai Stewart, Ectino- "Mlus (Cowlitzia) canalifer (Gabb), and Ficopsis hornii Gabb. The third stratigraphic unit of the Tejon is the Coldwater sandstone, .vhich in the area mapped is characterized by white, friable arkose sand­ stone, interbedded reddish sandy shale, and massive hard ledges com­ posed of numerous shells of Ostrea idricemis Gabb. The striking red and green beds of shale and the gritty white sandstone form the most distinctive features of the member. These colored beds show remark­ able continuity, having been traced more than 40 miles along the Santa Ynez Range westward from the type locality in Coldwater Canyon, where, according to Kew,5 they have the same lithologic features. The 3 W. L. W atts: Oil and gas yielding formations of California. Bull. 11, 1896; Bull. 19, 1900, Calif. State Mining Bureau. 4 The localities mentioned in this paper will be found on the Yentura, Santa Paula, Piru, and Mount Pinos sheets of the topographic maps of the United States published by the U. S. Geological Survey. 6 W. S. W. Kew: Geology and oil resources of a part of Los Angeles and Ventura counties, California. U. S. Geol. Survey Bull. 753, 1924, p. 28. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/39/4/1087/3414991/BUL39_4-1087.pdf by guest on 28 September 2021 STRATA AT MATILIJA 1091 Coldwater sandstone is about 2,500 feet thick near Matilija, and at several places it carries such Tejon fossils as Turritella uvasana Conrad, Venericardin hornii Gabb, and Peden calkinsi Arnold. , « SESPE FORMATION The continental Sespe formation, originally defined by Watts6 under the descriptive term “Sespe brownstone,” is about 4,000 feet thick in this region.