Douglas P. Smith

Semiannual patterns of erosion and deposition in upper Monterey Canyon from serial multibeam bathymetry

Recently acquired 3-m-resolution 244 kHz multibeam seafloor bathymetry (0.5 m depth precision) reveals geomorphology at sufficient detail to interpret small-scale features and short-term processes in the upper 4 km of Monterey Canyon, California. The study area includes the continental shelf and canyon features from 10 m to 250 m depth. The canyon floor contains an axial channel laterally bounded by elevated complex terrace surfaces. Sand waves with 2 m height and 35 m average wavelength dominate the active part of the canyon floor. The sand waves are strongly asymmetrical, indicating net down-canyon sediment transport in this reach. Terraces, including a broad 25-m-tall terrace complex near the head of the canyon, bear evidence of recent degradation of the canyon floor. Slump scars and gullies having a variety of sizes and relative ages shape the canyon walls. Serial georeferenced digital elevation models were analyzed to detect net changes in bathymetry or morphology occurring during both a six month period (September 2002 to March 2003) and a 24-h period (24 March to 25 March). Significant changes over the six month period include: (1) complete reorganization of the sand waves on the channel floor, (2) local channel degradation creating new 2-m-tall erosional terraces on the channel margins, (3) local channel widening that laterally eroded older channel margin terraces, and (4) 60 m extension of one minor gully head on a steep canyon wall. There were no discernable changes in morphology during the 24-h study period. Raster subtraction of serial bathymetric grids provides estimates of sediment erosion and deposition that occurred between the canyon head and a point 2 km down canyon during the six month study. Erosion of 320,000 m 3 (±80,000 m 3 ) of sediment occurred mainly in the tributaries, along the margins of the axial channel, and in the lowest 700 m of the analyzed reach. This eroded volume was approximately balanced by 260,000 m 3 (±70,000 m 3 ) of sediment deposition that was concentrated in the nearshore region along the rim of the canyon head. There was no measurable sediment gain or loss during the 24-h study period.

Evolutionary model for convergent margins facing large ocean basins: Mesozoic Baja California, Mexico

Mesozoic rocks of the Baja California Peninsula form a convergent-margin complex that is one of the best-preserved and longest-lived convergent-margin complexes in the world. It shows a three-phase evolutionary trend that we propose is typical of arc systems facing large ocean basins. The trend progresses from phase 1, highly extensional intraoceanic-arc systems, to phase 2, a mildly extensional fringing-arc system, to phase 3, a compressional continental-arc system. This trend is largely due to the progressively decreasing age of lithosphere that is sub- ducted. The modern Earth is strongly biased toward long-lived arc-trench systems, which are compressional, and so evolutionary models for convergent margins must be constructed from well-preserved ancient examples like Baja California.

Forearc-basin sedimentary response to rapid Late Cretaceous batholith emplacement in the Peninsular Ranges of southern and Baja California

The eastern Peninsular Ranges batholith is dominated by voluminous La Posta–type tonalite-granodiorite intrusions that compose half of the magmatic arc at present erosion level. Zircon U-Pb and hornblende 40 Ar/ 39 Ar results from these intrusions indicate that they were emplaced in a remarkably narrow interval (99–92 Ma) that closely followed cessation of west-directed compression of the arc system. Emplacement of the La Posta suite coincided with a major pulse of coarse-grained sediment into the adjacent forearc basin in early Cenomanian to middle Turonian time. Paleontologic control, and plutonic age and detrital zircon U-Pb data demonstrate the virtual absence of a time lag between magma emplacement and sedimentary response. The tight linkage between magmatism, arc exhumation, and sediment delivery to the forearc indicates that development of major erosional topography in the arc was driven by thermal and mechanical effects associated with large-volume batholith emplacement.

Garnet‐pyroxene‐amphibole xenoliths from Chino Valley, Arizona, and implications for continental lithosphere below the Moho

Garnet-pyroxene-amphibole xenoliths illustrate how P and T histories can be recorded in rocks from the crust-mantle transition and document the diversity of continental lithosphere below the Mohorovicic discontinuity. The xenoliths are from the Sullivan Buttes Latite in Chino Valley, Arizona, in the Transition Zone of the Colorado Plateau. The most definitive depth assignments depend upon garnet-pyroxene thermobarometry coupled with analysis of Ca and Al gradients in orthopyroxene. Websterites that record temperatures of 600 to 700°C contain orthopyroxene zoned in Al but not Ca, and these rocks were carried up from depths of at least 43 km. Websterites that record temperatures of 800–900°C contain more homogeneous orthopyroxene, and they were erupted from 70 to 80 km. Most eclogite and amphibole-rich xenoliths record temperatures in the range bracketed by websterites and so were probably erupted from similar depths. Element abundances and Sr, Nd, and Pb isotope ratios establish that protoliths of most xenoliths formed by crystal-melt fractionation from basaltic magmas. Diverse Sr and Nd isotopic compositions range from eNd ≈ +8 and 87Sr/86Sr ≈0.7045 for two Websterites to eNd ≈ −9 and 87Sr/86Sr ≈ 0.7064 for both parts of a composite eclogite. Most xenoliths probably have Proterozoic protoliths, although many record more recent thermal and metasomatic events, and a few probably formed from Cenozoic magmas. Observations are consistent with a reconstruction of the lithosphere in which eclogite and amphibole-rich rock were volumetrically important to depths of at least 70–80 km at 25 Ma. Anhydrous peridotite may not dominate just below the Mohorovicic discontinuity beneath Chino Valley or beneath some other localities on the Colorado Plateau and elsewhere. No evidence was observed in the Chino Valley suite for replacement of continental lithosphere during Phanerozic tectonism or for significant underplating in Cenozoic time.

Shallow magnetic inclinations in the Cretaceous Valle Group, Baja California: remagnetization, compaction, or terrane translation?

Paleomagnetic data from Albian to Turonian sedimentary rocks on Cedros Island, Mexico (28.2° N, 115.2° W) support the interpretation that Cretaceous rocks of western Baja California have moved farther northward than the 3° of latitude assignable to Neogene oblique rifting in the Gulf of California. Averaged Cretaceous paleomagnetic results from Cedros Island support 20 ± 10° of northward displacement and 14 ± 7° of clockwise rotation with respect to cratonic North America. Positive field stability tests from the Vizcaino terrane substantiate a mid-Cretaceous age for the high-temperature characteristic remanent magnetization in mid-Cretaceous strata. Therefore coincidence of characteristic magnetization directions and the expected Quaternary axial dipole direction is not due to post mid-Cretaceous remagnetization. A slump test performed on internally coherent, intrabasinal slump blocks within a paleontologically dated olistostrome demonstrates a mid-Cretaceous age of magnetization in the Valle Group. The in situ high-temperature natural remanent magnetization directions markedly diverge from the expected Quaternary axial dipole, indicating that the characteristic, high-temperature magnetization was acquired prior to intrabasinal slumping. Early acquisition of the characteristic magnetization is also supported by a regional attitude test involving three localities in coherent mid-Cretaceous Valle Group strata. Paleomagnetic inclinations in mudstone are not different from those in sandstone, indicating that burial compaction did not bias the results toward shallow inclinations in the Vizcaino terrane.

Mid-Cretaceous crustal extension recorded in deep-marine half-graben fill, Cedros Island, Mexico

Marine, mid-Cretaceous fore-arc strata on Cedros Island (Vargas and Pinos Formations) record the onset of extensional brittle deformation of the upper crust concomitant with initial uplift of blueschist-grade rock from great depth. The pre-extensional Vargas Formation is chiefly fine grained, and it contains facies interpreted as basin-plain, sand-rich-fan, and slope deposits that onlapped a Jurassic unconformity toward the north. The syn-extensional Pinos Formation is coarser grained than is the Vargas and records the initiation and filling of a northeast-southwest-trending, deep-marine, half-graben structure that formed by reactivation of a Jurassic fault zone located along the northern margin of the basin. The axis of the half graben acted as a submarine canyon that funneled gravelly sediment-gravity flows toward the south-west, whereas the shoulder of the half graben was draped by sandy turbidites and mud. A second episode of faulting on a fault parallel to, and synthetic with, the older half-graben fault produced a stress-transfer zone that steepened the intervening block, producing a catastrophic slope failure in the basin. Several points of evidence suggest that the canyon-bounding fault zone mapped at the northern margin of the basin was syndepositionally active. Basin-floor scarps are indicated by the presence of large, angular blocks derived from the Jurassic strata; these blocks occur within Cretaceous conglomerate and olistostromes, and as clast-supported breccia. Asymmetric basin subsidence is recorded in monotonic lithofacies trends that occur laterally away from the inferred fault zone. These trends include a decrease in average grain size and bed thickness, and an increase in interbedded mudstone and in lateral continuity of individual beds. Abundant slope-failure features, including slide blocks as much as 100 m wide by 40 m thick, also point to frequently recurring intrabasinal seismicity. The facies distribution and basin structures exposed on Cedros Island are analogous to aspects of the southern Viking Graben and the Ebro turbidite complex. Modern tectonic and physical analogues for the half graben are found at the Kuril margin and in the Aleutian are, where oblique convergence has produced fault-bounded submarine canyons of the same lateral and vertical scale as inferred for the Cretaceous basin on Cedros Island. The temporal coincidence between brittle deformation in the upper plate with peak blueschist metamorphism in the lower plate of the convergent margin suggests that initial blueschist uplift began soon after deepest burial.

Using Vessel-Based LIDAR to Quantify Coastal Erosion during El Nino and Inter-El Nino Periods in Monterey Bay, California

ABSTRACT Quan, S.; Kvitek, R.; Smith, D., and Griggs, G. 2013, Using vessel-based LIDAR to quantify coastal erosion during El Niño and inter-El Niño periods in Monterey Bay, California. Vessel-based light detection and ranging (LIDAR) was employed to collect coastal topography data and to quantify the rates of erosion and spatial distribution of coastal retreat around Monterey Bay, California during the 2008–09 (non-El Niño) and 2009–10 El Niño. These data were compared with pre/post-El Niño LIDAR data from 1997–98 to assess shoreline change and to test the following hypotheses: (1) that broad-scale (km) spatial distribution of erosion rates is positively correlated with wave energy, and (2) that fine-scale erosion hot spots (segments of the coastline exhibiting considerably higher rates of erosion than adjacent areas) shift at predictable alongshore wavelengths between consecutive El Niño and inter-El Niño periods. Broad-scale erosion was found to be significantly higher during the 2009–10 El Niño vs. the 2008–09 non-El Niño period in both the south (1.8 m vs. 0.1 m average) and north bays (0.5 m vs. 0.0 m average). The broad-scale distribution of erosion rates during the 2009–10 El Niño was positively correlated with wave energy. In southern Monterey Bay, erosion rates increased along a wave energy gradient from south to north, whereas erosion and wave energy were both focused and highest at a single location in the northern bay. Fine-scale erosion hot spots were found to occur during the 1997–98 and 2009–10 El Niño and the 1998–08 inter-El Niño period. These hot spots were found to be significantly correlated at −160 m during the 1997–98 El Niño to 1998–2009 inter-El Niño periods and 100 m during the 1998–2009 inter-El Niño to 2009–10 El Niño periods in southern Monterey Bay. Hot spots that occurred during one El Niño or inter-El Niño period shifted spatially alongshore during the subsequent El Niño or inter-El Niño period. Vessel-based LIDAR proved to be effective for detecting coastal change at high spatial resolutions and revealing fine-scale patterns of shoreline retreat.

New paleomagnetic, rock magnetic, and petrographic results from the Valle Group, Baja California, Mexico: Exploring the causes of anomalously shallow paleomagnetic inclinations

[1]xa0A detailed paleomagnetic, rock magnetic, and sedimentologic study was conducted on Cenomanian-age strata of the Valle Group at El Campito, Baja California, to examine the burial compaction effects on the paleomagnetic inclination of marine sedimentary rocks. Alternating field and thermal demagnetization isolated a characteristic remanence at 16 of 18 sites with a mean of Dec = 342.6°, Inc = 19.1°, and α95 = 3.4°. The inclination is 38.7° shallower than that predicted by the reference paleopole for North America. Laboratory experiments indicate that compaction could account for only 7.5° of the inclination shallowing. Fold tests at different scales yield both positive and negative results, making a tight constraint on the remanences age impossible. A primary postdepositional remanence is supported by bedding-parallel foliations in the anisotropy of anhysteretic remanence fabric. Although primary anomalously shallow inclinations could indicate significant southerly, then northerly, paleolatitudinal offset, a more likely scenario is a late Cenozoic low-temperature remagnetization, which is suggested by alteration along the edges of some detrital silicate grains and a bimodal magnetic grain size distribution revealed by isothermal remanence data. The synthesis of the data indicates that the Vizcaino terrane was contiguous with the Cordillera margin during remagnetization and underwent a subsequent 13.5 ± 5.4° clockwise rotation. We tentatively interpret that the remagnetization resulted from burial and/or Miocene volcanism, which caused an acquisition of thermochemical remanent magnetization (TCRM). This study reveals a complicated remanence that resists easy interpretation and suggests caution in studies of similar rocks.

Preservation Potential of Ultrabasic Volcanic Sand in an Arid Intracontinental Setting: Will the Hopi Buttes Maar-diatreme Field be Preserved in the Rock Record?

ABSTRACT Petrographic study of modern sand samples collected near Hopi Buttes volcanic field (northeastern Arizona) underscores the low preservation potential of mafic minerals and ultrabasic volcanic detritus even in an arid, intraplate tectonic setting. The Miocene-Pliocene Hopi Buttes volcanic field has been interpreted as a maar-diatreme and scoria cone field in which phreatomagmatic eruptions took place in a playa lake basin. The tephra, necks, dikes, flows, and maar crater fill are now sources of sediment found in local ephemeral streams. Once the volcanic field is eroded away, the main record of its existence will be as a provenance component in the sand it is now shedding. The locally derived volcanic components include monocrystalline pyroxene, glass shards, scoria, and a narrow range of ultrabasic volcanic rock fragments. Thin-section point counts (450 grains) on four sieved sand fractions from each of 7 sites downstream (0, 2, 4, 8, 12, 21, and 31 km) from one margin of the volcanic field provided the following results. (1) The proportion of volcanogenic to total sand grains decreases from an initial value of about 40% at a rate of about 0.8% per kilometer downstream from the source rocks over the first 30 km of stream length. (2) Dilution by sand eroded from other local source rocks, rather than physical grain-size reduction or chemical dissolution, drives the diminution of volcanogenic grain proportions. Detritus sourced from the Hopi Buttes volcanic field has a very low preservation/recognition potential, despite the arid climate and stable tectonic setting far removed from a plate margin. It follows that maar-diatreme volcanism is generally underrepresented in the sedimentary record.

Wetland Bathymetry and Mapping

Bathymetry is the measurement of underwater topography. In wetlands, development of bathymetric maps can have many applications, including determining water storage capacity and hydroperiod (depth and timing of flooding) of a wetland, assisting with wetland design and restoration and land use planning, and facilitating legal boundary determination. This chapter provides practical steps for mapping and modeling wadeable wetland bathymetry. By characterizing the bathymetry of wetlands, investigators can better understand key hydrologic, geomorphologic, and ecological processes of wetlands. Using standard survey equipment, investigators can plan and implement a relatively simple survey of wetlands. These data can be used to model and quantitatively analyze the surface area, volume, and bottom topography (bathymetry) of wetlands using standard geographic information system software.