The type Lospe Formation in the Casmalia Hills is an 800-m-thick sequence of sedimentary and minor volcanic rocks. The Lospe is entirely of early Miocene (Saucesian) age on the basis of palynomorphs, benthic foraminifers, and {sup 40}Ar/{sup 39}Ar ages of 17.70 {plus minus} 0.03 Ma (mean of seven determinations) and 17.39 {plus minus} 0.12 Ma (mean of six determinations). The {sup 40}Ar/{sup 39}Ar ages were measured on water-laid tuffs; these tuffs may have erupted from the same volcanic source as a welded tuff yielding an {sup 40}Ar/{sup 39}Ar age of 17.79 {plus minus} 0.10 Ma (mean of five determinations) from the Tranquillon volcanics on Tranquillon Mountain in the westernmost Transverse Ranges. Alluvial fan and fan-delta facies within the basal part of the Lospe are as thick as 200 m and consist mainly of conglomerate and sandstone derived from nearby fault-bounded uplifts of Mesozoic rocks. These coarse-grained facies grade upward into a sequence of interbedded sandstone and mudstone that accumulated in a shallow lake. Gypsum layers in the lake deposits contain sulfate depleted in {sup 34}S (0 to +3{per thousand}), suggesting that the sulfur had a hydrothermal origin. The uppermost 30 m of the Lospe consists of storm-deposited sandstone and mudstonemore » containing shallow-marine microfossils. The shallow-marine deposits are abruptly overlain by bathyal marine shale of the Point Sal Formation. The Lospe Formation records active faulting, volcanism, hydrothermal activity, and rapid subsidence during initial formation of the Neogene Santa Maria basin. These events may have resulted from crustal extension related to the beginning of clockwise rotation of the western Transverse Ranges about 18 to 17 Ma.« less
ABSTRACT The depositional history of the Eocene-Oligocene Burwash strike-slip basin is characterized by a transition from non-volcanic clastic sedimentation of the Amphitheatre Formation to deposition of lavas and volcaniclastic rocks of the overlying lower Wrangell volcanic sequence. The lowermost Wrangell volcanic deposits within the Burwash basin consist mostly of 110 m of trachybasalt, basaltic trachyandesite, and trachyte lava flows which directly overlie about 440 m of stream-dominated alluvial fan, fan-delta, and lacustrine deposits of the Amphitheatre Formation. Overlying the lava flows are about 50 m of interbedded volcaniclastic fluvial deposits, pumice-rich, andesitic pyroclastic-fallout deposits, and lithic-rich trachyandesitic pyroclastic-flow deposits. Although volcanism is known to occur within strike-slip basins, the role of volcanism in strike-slip basin evolution and the effects of volcanism on sedimentation in strike-slip basins are largely unknown. The depositional history of the sedimentary rocks (Amphitheatre Formation) and the volcanic rocks (Wrangell volcanic sequence) in the Burwash basin can be divided into four progressive stages: 1) progradational stream-dominated alluvial fan deposition; 2) lacustrine deposition; 3) renewed basin margin faulting, proximal alluvial fan deposition and initial volcanic vent formation; and 4) intrabasinal volcanism resulting in lava flow emplacement, pyroclastic eruptions, and fluvial volcaniclastic sedimentation. This study focuses on the complex interplay between volcanism and fluvial sedimentation that can be demons rated during stages 3 and 4, and on the overall reorganization of depositional systems in this strike-slip basin during the transition from non-volcanic clastic sedimentation to lava flows and volcaniclastic sedimentation. Paleocurrent data collected from the Burwash basin document major reorganization of paleodrainage during the clastic to volcaniclastic transition. Prior to volcanism, fluvial systems flowed westward toward and along the basin axis, whereas after the onset of volcanism fluvial systems drained northeastward, outward from the basin center. In addition, conglomerate compositions in the basin change at the clastic to volcaniclastic transition from extrabasinally-derived basement clasts eroded from uplifted source areas along the strike-slip basin margin to intrabasinally-derived volcanic clasts. Fluvial depositional style was also strongly influenced by contemporaneous explosive volcanism during the late stage of Burwash basin evolution. Fluvial aggradation, which occurred in the form of sand-dominated hyperconcentrated flood-flows and braided streams, was contemporaneous with episodes of pyroclastic activity. These syneruption fluvial deposits are sheet-form and contain abundant juvenile detritus (vitric grains and pumice), euhedral to subhedral and angular crystal fragments of plagioclase and hornblende, and a small percentage of basaltic lithic fragments. Fluvial degradation, marked by the incision of steep valleys, occurred during periods between eruptions. The inter-eruption fluvial deposits are characterized by lenticular beds of clast-supported conglomerate and trough- rossbedded sandstone and are composed predominantly of basaltic lithic grains with small amounts of crystal and vitric fragments. Subsequent eruptions generated pyroclastic flows which ponded in the fluvial valleys. These pyroclastic-flow deposits exhibit crystal-rich bases, lithic-rich lapilli-sized middle zones with out-sized lithic blocks and gas-escape structures, upper pumice-rich zones, and interbedded stratified surge deposits. Our results show that the onset of volcanism in the Burwash basin produced a major reorganization of fluvial drainage systems and sediment source areas. In addition, changes in fluvial depositional style were closely linked to alternating periods of explosive volcanic activity and quiescence during the late stages of strike-slip basin evolution. The excellent exposures in the Burwash basin provide an apt opportunity to examine the role of volcanism in the depositional history of an intracontinental strike-slip basin.
The early Miocene (22 Ma) Tecuya volcanic rocks of the San Emigdio Mountains were erupted during a regional episode of crustal extension in coastal California. These rocks are correlative with similar rocks in outcrops and the subsurface over at least 800 kmz in the southern San Joaquin basin. Initial dacitic eruptions produced laterally continuous subaerial to submarine pyroclastic flows. These facies rapidly buried alluvial fan deposits of the lower Tecuya Formation and marine sandstone of the Temblor Formation farther west. Following dacitic volcanism, massive basalt was deposited subaerially along the southeastern basin margin. Farther west, the basalt triples in thickness across synvolcanic, high-angle faults and consists of hyaloclastite and submarine basalt flows. Facies associations, thickness and tectural trends, and paleodispersal directions of the volcanic rocks were controlled by synvolcanic extensional faulting throughout the southern San Joaquin basin. The regional extent of these volcanic rocks provides an important chronostratigraphic marker and potential stratigraphic trap in a variety of depositional settings, from nonmarine to outer shelf and slope. Preliminary rare-earth element data indicate that both dacites and basalts exhibit LREE enrichment patterns. The {epsilon}{sub Nd}(O) values cluster around +3. Although these data are typical of island arc volcanic rocks, geological data clearlymore » indicate that the Tecuya volcanic rocks were erupted in an extensional tectonic setting near the continental margin. Some other model, perhaps involving crustal contamination of magmas produced along the subducted East Pacific Rise, followed by syntectonic eruption along normal faults, is needed to explain this enigmatic combination of geochemical and geological data.« less
This chapter reports new field, petrographic, and geochemical data for two Tertiary volcanic units and underlying sedimentary rocks in the southeast corner of the Mount McKinley 1:250,000-scale quadrangle. The volcanic units include the late Paleocene and early Eocene volcanic rocks of Foraker Glacier and the late Eocene and early Oligocene Mount Galen Volcanics. New 40 Ar/ 39 Ar dating on two samples from the lower part of the volcanic rocks of Foraker Glacier yield ages of 56.9±0.2 and 55.5±0.1 Ma. The volcanic rocks of Foraker Glacier unconformably overlie a 550-m-thick sequence of Late Cretaceous(?) sedimentary rocks that dip steeply north and unconformably overlie Paleozoic metamorphic rocks with a schistosity that dips steeply south. The sedimentary sequence includes a metamorphic-clast cobble-boulder alluvial-fan conglomerate overlain by fluvial and lacustrine conglomerate, sandstone, and mudstone. The coarse grain size and presence of bounding unconformities indicate that these sedimentary rocks fill a contractional basin and record pre-late Paleocene tectonic uplift of the adjacent Paleozoic metamorphic rocks. The overlying volcanic rocks of Foraker Glacier consist of a 200-m-thick interval of basalt and andesite lavas containing interbedded mudstone and volcaniclastic fluvial conglomerate overlain by 1,500 m of rhyolite lava and interbedded pyroclastic-flow deposits. The basalts are slightly depleted in incompatible trace elements (light rare-earth elements, Rb, Th, K) and along with the andesites have high Ba/Ta ratios (464–1,160). The rhyolites are strongly enriched in light rare-earth elements (LREEs), Rb, Th, and K, are depleted in Sr, P, and Ti, and have low Ba/Ta ratios (23–137), all of which indicate a combination of crustal assimilation and fractional crystallization in their petrogenesis. The Mount Galen Volcanics consists of basalt, andesite, dacite, and rhyolite lavas and dacite and rhyolite tuff and tuff-breccia. New 40 Ar/ 39 Ar dating of a basaltic andesite flow
ABSTRACT The Plush Ranch Formation (upper Oligocene and lower Miocene) consists of more than 1800 m of nonmarine sedimentary and volcanic rocks that record the history of an extensional basin referred to here as the Plush Ranch basin. Distinctive depositional facies, provenance, and sediment transport directions along each basin margin suggest an asymmetric basin shape that is consistent with a half-graben origin. The northern basin margin consists of sandstone-dominated alluvial-plain deposits (0.1-1.5 m thick, normally graded, lenticular sandstone beds). Small deltaic sequences 1-2 m thick were formed where these alluvial systems flowed southward into a lake. Lenses of massive, boulder-rich granitic breccia that represent rockslide deposits derived from a nearby northern granitic provenance nterfinger with the alluvial-plain facies. In contrast to the northern margin, the southern basin margin is represented by coarse-grained fan-delta deposits. Matrix- and clast-supported lenticular conglomerate beds 0.2-5 m thick with interbedded trough-cross-bedded pebbly sandstone represent braided-stream and flood-flow and/or noncohesive debris-flow deposits of alluvial fans that drained a highland area to the south. The alluvial-fan deposits interfinger to the north with several types of subaqueous sediment-gravity-flow facies including turbidite sandstone beds and matrix-supported debris-flow conglomerate. Each of the basin-margin depositional systems grades basinward and to the east into lacustrine deposits that include organic-rich dark shale, evaporite, and limestone. The lacustrine deposits represent the central and eastern parts of the Plush Ranch basin, which received little coarse siliciclastic sediment. Basalt deposits that are at least 50 m thick in the west and thicken eastward are interbedded mainly with the lacustrine facies. The southern margin of the Plush Ranch basin formed along a north-dipping, normal-slip fault along which dip separation increased toward the southwest; the northern margin developed on the tilted hanging-wall block of this fault. This fault was later reactivated in post-middle Miocene time as the present left-lateral strike-slip Big Pine fault. The Plush Ranch is one of several extensional and transtensional basins that formed in southern California and western Arizona about 25-20 Ma as a response to the change from a convergent to a strikeslip tectonic regime along western North America.
ABSTRACT Late Palaeocene uplift of the Beartooth Range in northwestern Wyoming and southwestern Montana generated the Beartooth Conglomerate along the eastern and northeastern flanks of the range. Systematic unroofing sequences and intraformational unconformities, folds, and faults in the conglomerate attest to deposition during uplift. Along the eastern flank, at least three ancient alluvial‐fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies. The two southern fans consist of 700+ m of sedimentary‐clast conglomerate and subordinate sandstone, dominated by hyperconcentrated‐flow and stream‐flow facies. The next fan to the north is dominated by plutonic and metamorphic clasts and contains abundant mud‐matrix‐supported debris‐flow facies, as well as stream‐flow facies. The northernmost depositional system consists of arkosic, channellized fluvial conglomerate and sandstone, overbank mudstone, and crevasse‐splay sandstone units. Palaeocurrent data indicate eastward dispersal, away from the Beartooth Range. Outstanding exposure of the Beartooth Conglomerate allows facies to be mapped on lateral photographic mosaics. A seven‐fold hierarchy of bounding surfaces and enclosed lithosomes exists in the Beartooth Conglomerate. First‐ through fourth‐order surfaces are analogous to first‐ through fourth‐order surfaces that recently have been documented in sandy fluvial facies, with one exception: sediment gravity flows are bounded by first‐order surfaces. Fifth‐order surfaces are either erosional (e.g. lateral migration of fanhead trench) or accretionary (e.g. aggradation of fan surface during backfilling of trench, and construction of lobes on lower fan during entrenchment on upper fan). Some fifth‐order surfaces coincide with intraformational angular unconformities and are thus the result of long‐term fanhead entrenchment following uplift of the upper part of the fan. Sixth‐order surfaces bound individual fan packages that are several hundred metres thick and ∼ 10 km 2 in area. The enclosed sixth‐order lithosomes are distinguishable in terms of petrofacies and lithofacies. A single seventh‐order surface bounds the entire Beartooth Conglomerate. Lower‐order lithosomes are produced by intrinsic processes of fan construction. Fifth‐order lithosomes can be attributed to both extrinsic and intrinsic controls. Sixth‐ and seventh‐order lithosomes are generated by extrinsic controls.
Abstract The Alaska Range suture zone exposes Cretaceous to Quaternary marine and nonmarine sedimentary and volcanic rocks sandwiched between oceanic rocks of the accreted Wrangellia composite terrane to the south and older continental terranes to the north. New U-Pb zircon ages, 40Ar/39Ar, ZHe, and AFT cooling ages, geochemical compositions, and geological field observations from these rocks provide improved constraints on the timing of Cretaceous to Miocene magmatism, sedimentation, and deformation within the collisional suture zone. Our results bear on the unclear displacement history of the seismically active Denali fault, which bisects the suture zone. Newly identified tuffs north of the Denali fault in sedimentary strata of the Cantwell Formation yield ca. 72 to ca. 68 Ma U-Pb zircon ages. Lavas sampled south of the Denali fault yield ca. 69 Ma 40Ar/39Ar ages and geochemical compositions typical of arc assemblages, ranging from basalt-andesite-trachyte, relatively high-K, and high concentrations of incompatible elements attributed to slab contribution (e.g., high Cs, Ba, and Th). The Late Cretaceous lavas and bentonites, together with regionally extensive coeval calc-alkaline plutons, record arc magmatism during contractional deformation and metamorphism within the suture zone. Latest Cretaceous volcanic and sedimentary strata are locally overlain by Eocene Teklanika Formation volcanic rocks with geochemical compositions transitional between arc and intraplate affinity. New detrital-zircon data from the modern Teklanika River indicate peak Teklanika volcanism at ca. 57 Ma, which is also reflected in zircon Pb loss in Cantwell Formation bentonites. Teklanika Formation volcanism may reflect hypothesized slab break-off and a Paleocene–Eocene period of a transform margin configuration. Mafic dike swarms were emplaced along the Denali fault from ca. 38 to ca. 25 Ma based on new 40Ar/39Ar ages. Diking along the Denali fault may have been localized by strike-slip extension following a change in direction of the subducting oceanic plate beneath southern Alaska from N-NE to NW at ca. 46–40 Ma. Diking represents the last recorded episode of significant magmatism in the central and eastern Alaska Range, including along the Denali fault. Two tectonic models may explain emplacement of more primitive and less extensive Eocene–Oligocene magmas: delamination of the Late Cretaceous–Paleocene arc root and/or thickened suture zone lithosphere, or a slab window created during possible Paleocene slab break-off. Fluvial strata exposed just south of the Denali fault in the central Alaska Range record synorogenic sedimentation coeval with diking and inferred strike-slip displacement. Deposition occurred ca. 29 Ma based on palynomorphs and the youngest detrital zircons. U-Pb detrital-zircon geochronology and clast compositional data indicate the fluvial strata were derived from sedimentary and igneous bedrock presently exposed within the Alaska Range, including Cretaceous sources presently exposed on the opposite (north) side of the fault. The provenance data may indicate ∼150 km or more of dextral offset of the ca. 29 Ma strata from inferred sediment sources, but different amounts of slip are feasible. Together, the dike swarms and fluvial strata are interpreted to record Oligocene strike-slip movement along the Denali fault system, coeval with strike-slip basin development along other segments of the fault. Diking and sedimentation occurred just prior to the onset of rapid and persistent exhumation ca. 25 Ma across the Alaska Range. This phase of reactivation of the suture zone is interpreted to reflect the translation along and convergence of southern Alaska across the Denali fault driven by highly coupled flat-slab subduction of the Yakutat microplate, which continues to accrete to the southern margin of Alaska. Furthermore, a change in Pacific plate direction and velocity at ca. 25 Ma created a more convergent regime along the apex of the Denali fault curve, likely contributing to the shutting off of near-fault extension-facilitated arc magmatism along this section of the fault system and increased exhumation rates.
'/%3e%3cuse xlink:href='%23a' transform='rotate(60)'/%3e%3ccircle r='17' fill='%23000095'/%3e%3ccircle r='15' fill='%23fff'/%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)