A. Eugene Fritsche

Chicxulub impact ejecta from Albion Island, Belize

Impact ejecta from the Albion Formation are exposed in northern Belize. The ejecta come from the outer portion of the continuous ejecta blanket of the Chicxulub crater, which is located 360 km to the northwest. The basal unit of the Albion Formation is a ∼1-m-thick clay and dolomite spheroid bed composed of up to four discrete flows. The clay spheroids are altered impact glass, and the dolomite spheroids are accretionary lapilli. The upper unit is a ∼15-m-thick coarse diamictite bed containing altered glass, large accretionary blocks, striated, polished, and impacted cobbles, and rare shocked quartz. The abundance of accretionary clasts, evidence for atmospheric drag sorting, and the presence of multiple flows in the Albion Formation indicate that atmospheres play an important role in the formation of the outer portions of continuous ejecta blankets of large craters.

Geology of the Inyo Mountains Volcanic Complex: Implications for Jurassic paleogeography of the Sierran magmatic arc in eastern California

An ∼3.1-km-thick volcanic complex exposed in the southern Inyo Mountains, east-central California, records Jurassic subaerial depositional environments along the east flank of the Sierran arc. This complex, which we name the Inyo Mountains Volcanic Complex, is subdivided into lower, middle, and upper stratigraphic intervals. The 200–580-m-thick lower interval comprises predominantly epiclastic strata deposited on alluvial fans and adjacent river flood plains that were inclined northeast. Mafic lava flows and rare reworked tuff in this interval record the onset of Jurassic(?) volcanism in this part of the arc. The 300–700-m-thick middle interval is composed predominantly of intermediate to silicic lava flows and tuffs representing a major episode of volcanism ending at ca. 169 Ma that is contemporaneous with emplacement of numerous plutons in the region. The >2260-m-thick upper interval is composed of epiclastic strata with minor intercalations of volcanic rock. Most of this interval accumulated on low-gradient flood plains that hosted evaporative lakes and that were episodically invaded by alluvial fan complexes. Three new U-Pb age determinations constrain the lower half of the upper interval to have been deposited during the interval from ca. 169 Ma to 150 Ma. The uppermost part of the complex remains undated but probably accumulated prior to 140 Ma. The Inyo Mountains Volcanic Complex is part of a belt of volcanic complexes that are the easternmost preserved Jurassic complexes of the Sierran arc. These complexes share sufficient similarities to suggest that they represent a distinctive arc-flank depositional province significantly different from that represented by coeval volcanic complexes preserved in roof pendants farther west, closer to the magmatic axis of the arc. Similarities among arc-flank complexes include predominantly to exclusively subaerial settings, substantial (>30%) portions of epiclastic strata, and existence at times of north- to northeast-inclined paleoslopes. We infer on the basis of the varying types and amounts of volcanic rocks that whereas most complexes in the arc-flank province were rarely if ever proximal to major eruptive centers, complexes in two areas (White Mountains and eastern Mojave Desert) were at times located in or adjacent to such centers. These differences lead us to speculate that the east flank of the Jurassic arc consisted of eastward-projecting volcanic salients separated by arc recesses—typified by the Inyo Mountains area—in which epiclastic deposition was dominant.

Miocene Paleogeography of Southwestern California and Its Implications Regarding Basin Terminology

A new palinspastic reconstruction for the latest Oligocene of southwestern California is created by: (1) backsliding the eastern part of the western Transverse Ranges west of the curved San Gabriel-Sierra Madre fault in a counterclockwise, rotational, left-slip direction in order to close the gap created by medial Miocene transrotational extension of the easternmost Los Angeles basin; and (2) bending the rocks in the westernmost Transverse Ranges, west of the Santa Monica Mountains, through an additional counterclockwise backrotation until they are in contact with the present southern California coast. This reconstruction restores Eocene forearc basin deposits of the southern California coast and the western Transverse Ranges regions into straight alignment with Eocene forearc-basin deposits to the north in the San Rafael Mountains. During the Oligocene, this forearc basin was divided into two depositional basins that received mostly nonmarine deposits. The northern of the two basins is named the DiSoCuMa...

Seastacks Buried Beneath Newly Reported Lower Miocene Sandstone, Northern Santa Barbara County, California: ABSTRACT

Three large, isolated exposures of a light-gray, coarse-grained, thick-bedded sandstone unit occur in the northern San Rafael Mountains of Santa Barbara County, California. These rocks are moderately fossiliferous and contain Vertipecten bowersi, Amussiopecten vanvlecki, Aequipecten andersoni, Otrea howelli, shark teeth, whale bones, and regular echinoid spines. The fossils indicate that the sandstone unit, although previously reported as upper(?) Miocene, correlates best with the lower Miocene Vaqueros Formation. This unit was deposited in angular unconformity on a Cretaceous, greenish-gray turbidite sequence of interbedded sandstone and shale, and onlaps the unconformity erosion surface from west to east, the unit being thicker in the west and older at its base. The underlying Cretaceous sandstone beds are well indurated, and during the eastward transgression of the early Miocene sea, they resisted wave erosion and stood as seastacks offshore of the advancing coastline, thus creating a very irregular topographic surface upon which the Vaqueros Formation was deposited. Some seastacks were as much as 4 m tall, as indicated by inliers of Cretaceous rock surrounded by 4-m thick sections of the Vaqueros Formation. End_of_Article - Last_Page 665------------

Neogene Drape Folding Over Pre-Neogene Flexural-Slip Movements in Western Transverse Ranges, California: ABSTRACT

In several locations in the western Transverse Ranges of California are folded Neogene sedimentary sequences that unconformably overlie homoclinal sequences of pre-Neogene rocks. To accomplish the folding of the rocks above the unconformity without apparent deformation of those below the unconformity, a mechanism other than simple crustal shortening is required. It is proposed that differential flexural slip along bedding planes in the limbs of large-amplitude pre-Neogene folds produced drape folds of small amplitude in the unconformably overlying Neogene rocks. This drape mechanism implies that the Neogene rocks were folded while they were still in the soft-sediment stage and that they were lengthened parallel to bedding during the process. Procedures that use the length of folded beds to determine the amount of crustal shortening, therefore, may indicate a greater amount of crustal shortening than actually occurred. End_of_Article - Last_Page 665------------

Preliminary Middle Tertiary Paleogeographic Maps of Area Represented by Two-Degree Los Angeles Map Sheet, California: ABSTRACT

Recent work by graduates of California State University, Northridge, serves as the basis for constructing six preliminary middle-Tertiary paleogeographic maps of the area represented by the two-degree Los Angeles map sheet of California. The End_Page 927------------------------------ maps are made partially palinspastic by a limited restoration of rocks along the San Gabriel and Big Pine faults to the positions occupied during the time represented by each map. The maps show that the area was divided into two depositional basins by the northwest-trending San Rafael highland. Fluvial deposition occurred in both basins during the Oligocene. In the northeastern Cuyama-Soledad basin, Oligocene-Miocene marine deposits transgressed eastward over a large delta. In the southwestern Ventura basin, marine transgression was from the southwest. In the late early Miocene the ocean breached the San Rafael highland and created a strait between the remaining San Rafael peninsula and the newly formed Ynez island. During the medial Miocene, marine transgression continued, further co necting the two basins into one and shrinking the size of Ynez island. Slight marine regression in the southeast at this time was caused by tectonic uplift in the region of the present-day Simi Hills. During the latest middle Miocene, movement occurred on the San Gabriel fault, thus isolating the Soledad basin and creating an inland lake. With continued fault movement, this lake moved southward during late Miocene and merged with the ocean creating a large estuary. Marine transgression continued in the southeast during the late Miocene, while marine regression occurred in the northwest. End_of_Article - Last_Page 928------------

Structure of Miocene Rocks in Sierra Madre, Northeastern Santa Barbara County, California: ABSTRACT

Folds in Miocene rocks of the central Sierra Madre commonly are northwest-trending, curvilinear, concentric, and symmetrical; asymmetry and overturning occur in the vicinity of reverse faults. Fold hinges undulate across the area and in places bifurcate or merge. Disharmonic folds and sandstone dikes occur locally in the Monterey Shale. Faults are of three types: (1) along the northeastern edge of the range is a distributive fault zone consisting of parallel, longitudinal, reverse-slip faults which join both westward and downward and which have a subsurface distributive component that is now obscured by more recent deposits on the northeast; (2) southwest of the reverse-slip faults is a group of parallel, longitudinal, normal-separation faults; and (3) the remaining faults are mostly small, diagonal faults of different types; many are hinged at one or both ends and have slips or separations that indicate contemporaneous folding and faulting. Folding began during late Miocene or Pliocene while Miocene sediment was still unconsolidated and water laden. Diagonal faulting, intrusion of sandstone dikes, and possibly disharmonic folding accompanied early deformation. Asymmetrical and overturned folds formed later and large reverse-slip faults developed along their hinges. Normal-separation faults possibly formed last as lag faults. Northeast-southwest shortening of the area averages about 22%. End_of_Article - Last_Page 433------------