Robert H. Osborne

Self-similar cataclasis in the formation of fault gouge

Particle-size distributions have been determined for gouge formed by the fresh fracture of granodiorite from the Sierra Nevada batholith, for Pelona schist from the San Andreas fault zone in southern California, and for Berea sandstone from Berea, Ohio, under a variety of triaxial stress states. The finer fractions of the gouge derived from granodiorite and schist are consistent with either a self-similar or a logarithmic normal distribution, whereas the gouge from sandstone is not. Sandstone gouges are texturally similar to the disaggregated protolith, with comminution limited to the polycrystalline fragments and dominantly calcite cement. All three rock types produced significantly less gouge at higher confining pressures, but only the granodiorite showed a significant reduction in particle size with increased confining pressure. Comparison with natural gouges showed that gouges in crystalline rocks from the San Andreas fault zone also tend to be described by either a self-similar or log-normal particle distribution, with a significant reduction in particle size with increased confining pressure (depth). Natural gouges formed in porous sandstone do not follow either a self-similar or a log-normal distribution. Rather, these are represented by mixed log-normal distributions. These textural characteristics are interpreted in terms of the suppression of axial microfracturing by confining pressure and the accommodation of finite strain by scale-independent comminution.

Cataclastic rocks of the San Gabriel fault—an expression of deformation at deeper crustal levels in the San Andreas fault zone

The San Gabriel fault, a deeply eroded late Oligocene to middle Pliocene precursor to the San Andreas, was chosen for petrologic study to provide information regarding intrafault material representative of deeper crustal levels. Cataclastic rocks exposed along the present trace of the San Andreas in this area are exclusively a variety of fault gouge that is essentially a rock flour with a quartz, feldspar, biotite, chlorite, amphibole, epidote, and Fe-Ti oxide mineralogy representing the milled-down equivalent of the original rock (Anderson and Osborne, 1979; Anderson et al., 1980). Likewise, fault gouge and associated breccia are common along the San Gabriel fault, but only where the zone of cataclasis is several tens of meters wide. At several localities, the zone is extremely narrow (several centimeters), and the cataclastic rock type is cataclasite, a dark, aphanitic, and highly comminuted and indurated rock. The cataclastic rocks along the San Gabriel fault exhibit more comminution than that observed for gouge along the San Andreas. The average grain diameter for the San Andreas gouge ranges from 0.01 to 0.06 mm. For the San Gabriel cataclastic rocks, it ranges from 0.0001 to 0.007 mm. Whereas the San Andreas gouge remains particulate to the smallest grain-size, the ultra-fine grain matrix of the San Gabriel cataclasite is composed of a mosaic of equidimensional, interlocking grains. The cataclastic rocks along the San Gabriel fault also show more mineralogiec changes compared to gouge from the San Andreas fault. At the expense of biotite, amphibole, and feldspar, there is some growth of new albite, chlorite, sericite, laumontite, analcime, mordenite (?), and calcite. The highest grade of metamorphism is laumontite-chlorite zone (zeolite facies). Mineral assemblages and constrained uplift rates allow temperature and depth estimates of 200 ± 30°C and 2–5 km, thus suggesting an approximate geothermal gradient of ~50°C/km. Such elevated temperatures imply a moderate to high stress regime for the San Andreas, which is consistent with experimental rock failure studies. Moreover, these results suggest that the previously observed lack of heat flow coaxial with the fault zone may be the result of dissipation rather than low stress. Much of the mineralogy of the cataclastic rocks is still relict from the earlier igneous or metamorphic history of the protolith; porphyroclasts, even in the most deformed rocks, consist of relict plagioclase (oligoclase to andesine), alkali feldspar, quartz, biotite, amphibole, epidote, allanite, and Fe-Ti oxides (ilmenite and magnetite). We have found no significant development of any clay minerals (illite, kaolinite, or montmorillonite). For many sites, the compositions of these minerals directly correspond to the mineral compositions in rock types on one or both sides of the fault. Whole rock major and trace element chemistry coupled with mineral compositions show that mixing within the zone of cataclasis is not uniform, and that originally micaceous foliated, or physically more heterogeneous rock units may contribute a disproportionally large amount to the resultant intrafault material. As previously found for the gouge along the San Andreas, chemical mobility is not a major factor in the formation of cataclastic rocks of the San Gabriel fault. We see only minor changes for Si and alkalies; however, there is a marked mobility of Li, which is a probable result of the alteration and formation of new mica minerals. The gouge of the San Andreas and San Gabriel faults probably formed by cataclastic flow. There is some indication, presently not well constrained, that the fine-grained matrix of the cataclasite of from the San Gabriel fault formed in response to superplastic flow.

PLIO-PLEISTOCENE LACUSTRINE STROMATOLITES FROM LAKE TURKANA, KENYA: MORPHOLOGY, STRATIGRAPHY AND STABLE ISOTOPES

Abell, P.I., Awramik, S.M., Osborne, R.H. and Tomellini, S., 1982. Plio-Pleistocene lacustrine stromatolites from Lake Turkana, Kenya: morphology, stratigraphy and stable isotopes. Sediment. Geol., 32: 1--26. A sequence of fossil stromatolites from Lake Turkana in Kenya was examined for 51SO and 513C content. These stromatolites, ranging in age from Holocene (~10,000 yrs B.P.) to Middle Pliocene (-3 m.y.) showed a variety of growth forms from oncolitic, columnar layered to bulbous heads. The stromatolites used in our study contain filamentous blue-green algae of one morphological type and rare coccoids; thus the stromatolites are considered biogenic. The stable isotope ratios for oxygen and carbon indicate changing climatic conditions, ranging from a cool, wet climate prior to ca. 1.9 m.y. to much drier, warmer conditions around 1.4 m.y., followed in turn by a somewhat cooler and wetter climate at the end of the Pleistocene.

Modern lacustrine stromatolites, Walker Lake, Nevada

Abstract The Walker River drainage basin occupies about 10,000 km2 in western Nevada and parts of California and is essentially a closed hydrologic system which drains from the crest of the Sierra Nevada in California and terminates in Walker Lake, Nevada. Walker Lake trends north and is about 27.4 km long and 8 km wide with water depths exceeding 30.5 m. The lake is situated in an asymmetric basin with steep alluvial fans flanking the western shoreline (Wassuk Range) and more gentle but areally more extensive alluvial fans flanking the eastern shoreline (Gillis Range). Exposed lake terraces and the present shoreline of Walker Lake record a sequence of Pleistocene and Holocene stromatolitic and tufaceous carbonate deposits. Small generalized and columnar stromatolites, frequently encrusted on exposed coarse-grained clasts or bedrock, are present along parts of the nearshore margin of Walker Lake and at elevated lake stands. Columnar stromatolites as much as 4 cm high are subcylindrical to club shaped discrete, and laterally linked at the base with local branching. These digitate stromatolites start as wavy, generalized stromatolites which are vertically transitional to small, laterally linked cabbage heads with laminae which thicken over the crests. Although algal structures are not well preserved in the older stromatolites, recent precipitation of low magnesium calcite occurs as smooth encrustations and as tiny mounds which are consistently associated with a diverse, seasonally variable, green and blue-green algal community including Cladophora glomerata, Ulothrix (cf. aequalis), Gongrosira, Schizothrix, Amphithrix janthina, Calothrix, Homeothrix, Spirulina, Anabaena, Lyngbya, and Entophysalis. Cladophora glomerata and a species of Ulothrix, which are the two most abundant algae within the Walker Lake stromatolite community, are known to condition semi-alkaline lake water by the removal of CO2 from bicarbonate during photosynthesis. Such conditioning results in the precipitation of calcium carbonate, which is trapped and bound by an understory of green and blue-green algae. The occurrence of stromatolites in highly siliciclastic lakes seems to be restricted to shoreline and nearshore environments, and can be used to locate ancient lake margins.

Petrogenesis of cataclastic rocks within the San Andreas fault zone of Southern California U.S.A.

Abstract This paper petrologically characterizes cataclastic rocks derived from four sites within the San Andreas fault zone of southern California. In this area, the fault traverses an extensive plutonic and metamorphic terrane and the principal cataclastic rock formed at these upper crustal levels is unindurated gouge derived from a range of crystalline rocks including diorite, tonalite, granite, aplite, and pegmatite. The mineralogical nature of this gouge is decidedly different from the “clay gouge” reported by Wu (1975) for central California and is essentially a rock flour with a quartz, feldspar, biotite, chlorite, amphibole, epidote and oxide mineralogy representing the milled-down equivalent of the original rock. Clay development is minor (less than 4 wt. %) to nonexistent and is exclusively kaolinite. Alterations involve hematitic oxidation, chlorite alteration on biotite and amphibole, and local introduction of calcite. Electron microprobe analysis showed that in general the major minerals were not reequilibrated with the pressure—temperature regime imposed during cataclasis. Petrochemically, the form of cataclasis that we have investigated is largely an isochemical process. Some hydration occurs but the maximum amount is less than 2.2% added H 2 O. Study of a 375 m deep core from a tonalite pluton adjacent to the fault showed that for Si, Al, Ti, Fe, Mg, Mn, K, Na, Li, Rb, and Ba, no leaching and/or enrichment occurred. Several samples experienced a depletion in Sr during cataclasis while lesser number had an enrichment of Ca (result of calcite veining). Texturally, the fault gouge is not dominated by clay-size material but consists largely of silt and fine sand-sized particles. An intriguing aspect of our work on the drill core is a general decrease in particulate size with depth (and confining pressure) with the predominate shifting sequentially from fine sand to silt-size material. The original fabric of these rocks is commonly not disrupted during the cataclasis. It is evident that the gouge development in these primarily igneous crystalline terranes is largely an in situ process with minimal mixing of rock types. Fabric analyses reveal that brecciation (shattering), not shearing, is the major deformational mechanism at these upper crustal levels.

Late Quaternary history of the Ventura mainland shelf, California

Abstract The Ventura mainland shelf off California provides an interesting example of sedimentary response to tectonic activity and eustatic sea-level fluctuations during the late Quaternary. A network of 370 km of high-resolution seismic-reflection profiles provides the basis for the study. The high local rates of uplift, as well as local basin subsidence, have resulted in two distinct styles of sedimentation. A late Pleistocene angular unconformity separates folded Plio-Pleistocene strata from undeformed upper Pleistocene and Holocene strata on the northern shelf. In contrast, the southern shelf is characterized by a thick section of stacked, progradational deltaic sequences containing a well-preserved record of late Quaternary sedimentation. Sediment overlying the unconformity north of the Pitas Point fault ranges from 10 to 40 m in thickness, whereas the equivalent sequence in the adjacent southern shelf is 80–85 m thick. The distribution of upper Pleistocene and Holocene deposits is controlled by structural features including the Pitas Point and Oak Ridge faults, and the Ventura Avenue anticline, as well as proximity to fluvial input from the Santa Clara and Ventura Rivers. Within upper Pleistocene deposits, a pair of buried marine terraces occur at depths of 33 and 50 m below present sea level (b.p.s.l.). Terrace deposits consist of a basal, coarse-grained, transgressive deposit which is transitional into a finer grained marine facies. These deposits are associated with a period of aggradation during minor late Wisconsinan sea-level cycles. Deltaic deposits beneath the southern shelf 4 km north of Hueneme submarine canyon suggest that the Santa Clara River debouched into the sea at this location during the late Pleistocene. The Santa Clara River shifted southward during the late Wisconsinan and cut a large channel, now buried, near Hueneme canyon. This channel indicates that relative sea level was at least 113 m b.p.s.l. during the late Wisconsinan. A smaller channel indicates a fall in sea level to 46 m b.p.s.l. during the Holocene. The Oak Ridge fault displays a general increase in offset seaward across the shelf, with a maximum vertical displacement of 17 m on the Saugus/upper Pleistocene contact. Displacements on the Pitas Point fault increase shoreward, attaining as much as 40 m of vertical displacement of the late Pleistocene unconformity. Nearshore, the Pitas Point fault vertically displaces the seafloor by 4 m.

Late Middle and early Late Ordovician history of the Cincinnati arch province, central Kentucky to central Tennessee

Six limestone classes occur in the Ordovician strata from Lexington, Kentucky, to Nashville, Tennessee. Sediments that make up classes 1 through 4 were deposited in “open” epicontinental marine environments, whereas classes 5 and 6 reflect more “restricted” environments. These classes were used to calculate relative mechanical energy and depth indices for a defined time-stratigraphic interval. The base of this interval is a bentonite bed that occurs near the base of the Brannon Member of the Lexington Limestone, and the top is defined by a major change in the relative abundance of platform conodonts. Relative mechanical energy and depth contours intersect the present axis of the Cincinnati arch at a high angle, which suggests that a continuous arch was not present. Shoal environments existed in the Lexington area and northeast of the present Nashville dome. These shoals correspond to the positions of the Lexington and Nashville domes or precursors to these domes which were present during the defined time interval. Paleobathymetric contours parallel east- and south-trending normal faults that are present in and south of the Lexington, Kentucky, area. This suggests that these faults were active during late Middle Ordovician time and were partly responsible for creating the bathymetric relief necessary for higher-energy carbonate sediments to accumulate. Deeper-water environments between the two shoal areas reflect the presence of the Rome trough. The general bathymetric trend for these strata is one of submergence.

Sources and Petrology of Beach Sand from Southern Monterey Bay, California

ABSTRACT Considerable local and international demand exists for beach sand from southern Monterey Bay, California. Uses of this specialty sand include filtration, sandblasting, foundry, and surface finishes. Approximately 90% is medium- to coarse-grained sand and granule gravel. Quartz, feldspar, and granitic rock fragments total 82 % by volume, which results in a Mohs hardness of about 6.5 and a high temperature of fusion. Sand in the 0.7 to 1.0 mm range has an average Folk roundness of 3.2 (subrounded) and is well polished. Its amber color, valuable for architectural purposes, is a product of iron-staining on the surfaces of 17% of the grains. Knowledge of the sand provenance provides a geologic basis for calculation of a new sand budget or re-evaluation of available budgets to determine the degree to which beach sand mining contributes to coastal erosion in southern Monterey Bay. Comparisons were made of grain-size distributions, lithologic compositions, and grain surface attributes to determine the provenance of medium- to coarse-grained beach sand. Landward migration of relict or modern offshore surface sand and wave erosion of pre-Flandrian and Flandrian coastal dunes are important sources, and there may be significant contributions from the Salinas River during flooding. Contributions by southward littoral transport across the head of Monterey Canyon, northward littoral transport from the Monterey Peninsula, offshore w nds, and landward migration from offshore exposures of the Monterey, Paso Robles, and Aromas Formations are negligible.

Sources and Nonsources of Beach Sand Along Southern Monterey Bay California--Fourier Shape Analysis

ABSTRACT The shoreline along the southern portion of Monterey Bay California is undergoing severe erosion. Industrial and architectural sand has been dredged from this portion of the bay for the last 70 years. This withdrawal of sand coupled with weak to absent longshore contributions of new sand has been blamed for modern beach erosion. To determine the relative importance of potential sources to the sediment budget, 30 samples of 130-300 grains each were compared using Fourier grain shape analysis. Potential sources (including Flandrian and preFlandrian dunes, Salinas River, and offshore sands) can be discriminated on the basis of grain shape. Comparison of data from the beach samples and samples from the mining operations shows that most beach sand, offshore (shallow water) sands, and the m ned sand are primarily (70%-90%) derived from Flandrian and preFlandrian dunes. North of the Salinas River, the beach is dominated by fluvial sediments. This study does not prove that sand mining is the major cause of erosion along the southern half of the bay. This analysis does demonstrate however, that recharge of sand onto the southern beaches from the Salinas River is not significant and so cannot be invoked to provide a mechanism to support the concept that the mined sand is a renewable resource.

Petrology of Late Precambrian-Cambrian Quartzose Sandstones in the Eastern Mojave Desert, Southeastern California

ABSTRACT Petrographic and radiographic analyses were conducted on oriented samples of Late Precambrian-Cambrian quartzose sandstones collected from five cratonal and five miogeoclinal stratigraphic sections in the eastern Mojave Desert, southeastern California. Twelve petrographic variables were measured in each of 494 samples. Cratonal sandstones are dominantly subarkose, whereas miogeoclinal sandstones are subarkose and quartzarenite in almost equal proportions. Sediments were derived chiefly from granitic rocks, gneiss, and schist as well as older sedimentary and metasedimentary rocks. The cratonal source areas were to the east and southeast. Samples collected from six northern sections within the Cordilleran frontal thrust belt have a higher content of undulatory quartz and silica cement and are partially recrystallized when compared with samples from four southern sections. Polynomial time-trend analysis showed considerable variation in mineral composition with thickness at each stratigraphic section. Areal variation of total quartz was estimated from analysis of variance models. The difference was not highly significant for the Stirling Quartzite. Depositional history was inferred for each formation on the basis of petrography, clasticity and sedimentary structures. The Johnnie Formation fines upward, which suggests a marine transgression or change from open to partially restricted shelf environments. The Stirling Quartzite coarsens upward and consists largely of prograding mature shelf sands or deposits of migrating bars and barrier beaches. The Wood Canyon Formation is dominantly subtidal with local evidence for intertidal deposition.