Papers by Christine Siddoway
Marine sediments, obtained from cores and captures from deep sea and continental shelf sites of W... more Marine sediments, obtained from cores and captures from deep sea and continental shelf sites of West Antarctica, contain rich records of latest Miocene to Present glacial and deglacial processes and conditions at the margin of the West Antarctic ice sheet (WAIS). The materials we are investigating were recovered from a) Resolution Drift on the Amundsen Sea continental rise (water depths >3900m), b)the continental shelf in the Amundsen Sea, Wrigley Gulf, and Sultzberger Bay (water depths <1000m). Resolution Drift cores were drilled by IODP Expedition 379 (Gohl et al., doi:10.14379/iodp.proc.379.2021) in sediments dominated by compacted clay and silty clay, with conglomeratic intervals of ice-rafted detritus (IRD) and downslope deposits. The shelf sediments were recovered by piston core, trigger core, and Smith McIntyre Grab (SMG) during USA research cruises of the RVIB Nathaniel B Palmer (1999, 2000, 2007) and USCGC Glacier (1983). The shelf samples are non-compacted clay, cont...
&lt;p&gt;IODP Expedition 379 to the Amundsen Sea continental rise recovered latest Miocen... more &lt;p&gt;IODP Expedition 379 to the Amundsen Sea continental rise recovered latest Miocene-Holocene sediments from two sites on a drift in water depths &gt;3900m. Sediments are dominated by clay and silty clay with coarser-grained intervals and ice-rafted detritus (IRD) (Gohl et al. 2021, doi:10.14379/iodp.proc.379.2021). Cobble-sized dropstones appear as fall-in, in cores recovered from sediments &gt;5.3 Ma.&amp;#160; We consider that abundant IRD and the sparse dropstones melted out of icebergs formed due to Antarctic ice-sheet calving events. We are using petrological and age characteristics of the clasts from the Exp379 sites to fingerprint their bedrock provenance. The results may aid in reconstruction of past changes in icesheet extent and extend knowledge of subglacial bedrock.&lt;/p&gt;&lt;p&gt;Mapped onshore geology shows pronounced distinctions in bedrock age between tectonic provinces of West or East Antarctica (e.g. Cox et al. 2020, doi:10.21420/7SH7-6K05; Jordan et al. 2020, doi.org/10.1038/s43017-019-0013-6). This allows us to use geochronology and thermochronology of rock clasts and minerals for tracing their provenance, and ascertain whether IRD deposited at IODP379 drillsites originated from proximal or distal Antarctic sources. We here report zircon and apatite U-Pb dates from four sand samples and five dropstones taken from latest Miocene, early Pliocene, and Plio-Pleistocene-boundary sediments. Additional Hf isotope data, and apatite fission track and &lt;sup&gt;40&lt;/sup&gt;Ar/&lt;sup&gt;39&lt;/sup&gt;Ar Kfeldspar ages for some of the same samples help to strengthen provenance interpretations.&lt;/p&gt;&lt;p&gt;The study revealed three distinct zircon age populations at ca. 100, 175, and 250 Ma. Using Kolmogorov-Smirnov (K-S) statistical tests to compare our new igneous and detrital zircon (DZ) U-Pb results with previously published data, we found strong similarities to West Antarctic bedrock, but low correspondence to prospective sources in East Antarctica, implying a role for icebergs calved from the West Antarctic Ice Sheet (WAIS). The ~100 Ma age resembles plutonic ages from Marie Byrd Land and islands in Pine Island Bay.&amp;#160; The ~250 and 175 Ma populations match published mineral dates from shelf sediments in the eastern Amundsen Sea Embayment as well as granite ages from the Antarctic Peninsula and the Ellsworth-Whitmore Mountains (EWM). The different derivation of coarse sediment sources requires changes in iceberg origin through the latest Miocene, early Pliocene, and Plio/Pleistocene, likely the result of changes in WAIS extent.&lt;/p&gt;&lt;p&gt;One unique dropstone recovered from Exp379 Site U1533B is green quartz arenite, which yielded mostly 500-625 Ma detrital zircons. In visual appearance and dominant U-Pb age population, it resembles a sandstone dropstone recovered from Exp382 Site U1536 in the Scotia Sea (Hemming et al. 2020, https://gsa.confex.com/gsa/2020AM/meetingapp.cgi/Paper/357276). K-S tests yield high values (P &amp;#8805; 0.6), suggesting a common provenance for both dropstones recovered from late Miocene to Pliocene sediments, despite the 3270 km distance separating the sites. Comparisons to published data, in progress, narrow the group of potential on-land sources to exposures in the EWM or isolated ranges at far south latitudes in the Antarctic interior.&amp;#160; If both dropstones originated from the same source area, they could signify dramatic shifts in the WAIS grounding line position, and the possibility of the periodic opening of a seaway connecting the Amundsen and Weddell Seas.&lt;/p&gt;
GSA Annual Meeting in Denver, Colorado, USA - 2016, 2016
Scientific Data
A dataset to describe exposed bedrock and surficial geology of Antarctica has been constructed by... more A dataset to describe exposed bedrock and surficial geology of Antarctica has been constructed by the GeoMAP Action Group of the Scientific Committee on Antarctic Research (SCAR) and GNS Science. Our group captured existing geological map data into a geographic information system (GIS), refined its spatial reliability, harmonised classification, and improved representation of glacial sequences and geomorphology, thereby creating a comprehensive and coherent representation of Antarctic geology. A total of 99,080 polygons were unified for depicting geology at 1:250,000 scale, but locally there are some areas with higher spatial resolution. Geological unit definition is based on a mixed chronostratigraphic- and lithostratigraphic-based classification. Description of rock and moraine polygons employs the international Geoscience Markup Language (GeoSciML) data protocols to provide attribute-rich and queryable information, including bibliographic links to 589 source maps and scientific l...
Abstracts with programs, 2022
Geological Society of America Abstracts with Programs
The Proterozoic-Phanerozoic transition is commonly marked in the geologic record by a substantial... more The Proterozoic-Phanerozoic transition is commonly marked in the geologic record by a substantial time gap across the "Great Unconformity". Classically, this feature refers to where Cambrian sedimentary units overlie crystalline basement on North America, but it has been globally correlated with unconformities elsewhere. Erosion across this feature has been linked to the Cambrian explosion, Earth's oxygenation, and the initiation of modern plate tectonics. "The Great Unconformity" has traditionally been referred to in the singular, conjuring up the notion of a synchronous global feature generated by one erosion event, such as Snowball Earth. However, unconformities in different places needn't have the same history and cause.
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey e... more This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or
Precambrian gneisses in the Highland Mountains of SW Montana hold information about the regional ... more Precambrian gneisses in the Highland Mountains of SW Montana hold information about the regional extent and effects of the Big Sky Orogeny, an upper-amphibolite grade tectonic event at ~1.77 Ga (Harms et al., 2004). Dynamic fabrics in high-temperature mylonitic rocks at Camp Creek and other sites offer the means to assess metamorphic conditions and kinematic shear sense during tectonism. Garnet, K-feldspar and aligned prismatic sillimanite suggest temperatures in excess of 650oC at Camp Creek. Dynamic fabrics are overprinted by randomly oriented prismatic sillimanite and muscovite, suggesting that metamorphism at moderate temperature outlasted deformation. Investigation of the textures may be used to test the applicability of the Big Sky orogeny pressure-temperature path formulated for the neighboring Tobacco Root Mountains (Cheney et al., 2004).
The Journal of Geology, 2021
In situ zircon U-Pb and Hf isotopic data from ∼1.1 Ga intrusive igneous rocks in Mexico and from ... more In situ zircon U-Pb and Hf isotopic data from ∼1.1 Ga intrusive igneous rocks in Mexico and from Grenvillian (0.9–1.3 Ga) detrital zircons in sandstones from the southern midcontinent of Laurentia were used to refine provenance determinations for the Grenvillian detrital zircons delivered to southwestern Laurentia from the Neoproterozoic to the Cambrian and to address the reduction in the Grenvillian detrital zircon abundances documented in Cambrian sandstones from this region. Igneous zircons from Mesoproterozoic anorthosites and granites in northern Sonora have low εHf(0) values (<−22) and could not have been sources of the higher-εHf(0) (>−22), ∼1.1 Ga detrital zircons characteristic of Ediacaran to Terreneuvian sandstones in southwestern Laurentia. Abundant Grenvillian detrital zircons in Cryogenian sandstone injectites from central Colorado have U-Pb ages and high εHf(0) values (>−22) similar to those of zircons in Ediacaran to Terreneuvian sandstones throughout southwestern Laurentia. These zircons were derived from Mesoproterozoic rocks in the Llano uplift and vicinity in Texas and were fluvially transported across southwestern Laurentia from the Cryogenian to the Terreneuvian. In contrast, Cambrian glauconitic sandstones in the subsurface of east-central Colorado and from the Sawatch Sandstone in central Colorado have low Grenvillian zircon abundances, as observed in Cambrian sandstones exposed farther west in Laurentia. The low abundances of Grenvillian detrital zircons in sandstones found both east (Colorado) and west of the strike of a proposed Cambrian “transcontinental arch” suggest that this feature did not disrupt the supply of Llano uplift–derived Grenvillian zircons to southwestern Laurentia. The low Grenvillian zircon abundances instead coincide with the development of marine conditions in south-central Laurentia, suggesting that the progressive encroachment of the Sauk Sea into the continental interior provided a sink for fluvial sediments derived from the Llano uplift that restricted their transport farther to the north and west in the continent.
Lithosphere, 2014
Detrital zircon provenance analysis is used to resolve the age of sandstone injectites together w... more Detrital zircon provenance analysis is used to resolve the age of sandstone injectites together with source sandstones that form faultbounded, tabular bodies within Mesoproterozoic crystalline rocks of the Colorado Front Range. Named Tava sandstone (informal), the unit is a product of liquefaction and remobilization of mature quartz sediment within source bodies having volumes ≥1 × 10 6 m 3 into dikes up to 6 m in width. To surmount the indeterminate age of emplacement, we obtained new U-Pb detrital zircon age data for two source sandstones, three dikes and one sill, for comparison to four Paleozoic arenites. Tava age distributions feature a dominant 1.33-0.97 Ga broad age group and narrow ca. 1.11, 1.44, and 1.70 Ga groups, with several smaller age groups >1.5 Ga. The Tava detrital zircon results are dissimilar to Paleozoic sandstones but closely resemble published detrital zircon reference data for Grenville orogen-derived siliciclastic units of the western United States. The similarity in age distributions is borne out by statistical comparisons among Tava sandstone, Paleozoic samples, and Neoproterozoic strata that reveal a high probability of correlation of Tava sandstone to ca. 800-680 Ma strata deposited during intracontinental extension. We conclude that Tava sandstone is Neoproterozoic in age and provides a new avenue to investigation of Rodinia's terrestrial paleoenvironment.
Lithosphere, 2010
In the Wet Mountains, Colorado, Proterozoic rocks exposed along an oblique north-south tilted sec... more In the Wet Mountains, Colorado, Proterozoic rocks exposed along an oblique north-south tilted section preserve evidence of regional deformation and high temperature metamorphism in the middle and lower crust at ca. 1435-1365 Ma. Deformation of gneisses in the northern Wet Mountains is partitioned within discrete zones of subvertical foliation and northeast-trending folds, a product of northwest-southeast contraction or constriction associated with transcurrent deformation. Gneisses in the north are generally not migmatitic, and granitic intrusions form discrete bodies with distinct contacts. Shear zone foliation is cut by a late syntectonic dike with a U-Pb zircon age of 1430+5/-3 Ma, constraining the age of shear zone deformation in the upper crust. In the central to southern Wet Mountains, gneisses exhibit migmatitic foliation that dips moderately northeast, with dip-to oblique-slip mineral lineation throughout. Granite forms pervasive sills and interconnected sheets with gradational or indistinct contacts. Gneissic granite that yields a U-Pb zircon age of 1435 ± 4 Ma was emplaced into amphibolite gneiss containing 1436 ± 2 Ma metamorphic zircon. Younger, foliated granite sills were emplaced at 1390 ± 10 Ma. Our new results indicate contemporaneous deformation and metamorphism throughout the middle and lower crust at ca. 1.4 Ga. We interpret the zone of migmatitic crust pervaded by granite to represent a weak, low-viscosity, fl owing lower crust that controlled the pattern of distributed deformation in the comparatively strong, brittle crust above. Thus, the Wet Mountains may be viewed as a deeply exhumed analog for the mid-crustal, low-viscosity layers that are inferred to exist in modern intracontinental orogenic settings and continental rift provinces.
Journal of Metamorphic Geology, 2011
The Fosdick migmatite-granite complex in West Antarctica records evidence for two high-temperatur... more The Fosdick migmatite-granite complex in West Antarctica records evidence for two high-temperature metamorphic events, the first during the Devonian-Carboniferous and the second during the Cretaceous. The conditions of each high-temperature metamorphic event, both of which involved melting and multiple melt-loss events, are investigated using phase equilibria modelling during successive melt-loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm-Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian-Carboniferous (M 1) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M 1 metamorphism (820-870°C, 7.5-11.5 kbar) and the formation of garnet-sillimanite-biotite-melt-bearing assemblages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M 1 conditions. The Cretaceous (M 2) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse-grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm-Nd ages for garnet are similar at 102-99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M 1 garnet constrains M 2 conditions to 830-870°C and 6-7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M 2 event, which would facilitate overprinting of M 1 compositions during the M 2 prograde metamorphism. Measured garnet compositions and Sm-Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm-Nd system during the Cretaceous M 1 overprint. The c. 102-99 Ma garnet Sm-Nd ÔclosureÕ ages correspond to cooling below 700°C during the rapid exhumation of the Fosdick migmatite-granite complex.
Gondwana Research, 2013
A diorite pluton and widely distributed mafic dykes occur in the Fosdick migmatite-granite comple... more A diorite pluton and widely distributed mafic dykes occur in the Fosdick migmatite-granite complex, which is interpreted to represent middle-to-lower crustal rocks of the paleo-Pacific active continental margin of Gondwana. The mafic dykes exhibit a variety of relationships with host rocks in the field ranging from undeformed dykes with sharp contacts with host gneisses to dismembered dykes with commingled textures and numerous back-veins of leucosome intruded from host migmatitic gneisses suggestive of significant interaction with crustal rocks. New U-Pb ages for magmatic zircon in these rocks yield Cretaceous crystallization ages ranging from ca 113 Ma to ca 98 Ma for the mafic dykes and ca 100 Ma for the diorite pluton. These mafic intrusive rocks, which contain abundant hydrous minerals, are medium-to high-K-series calc-alkaline rocks with basic-intermediate compositions (47-59 wt.% SiO 2 for the mafic dykes and 52-56 wt.% SiO 2 for the diorite pluton). They have trace element patterns characterized by LILE enrichments and negative Nb anomalies indicating an origin from a hydrous mantle source metasomatized by slab-derived components. The samples without evidence of interaction with crustal rocks (11 of 14 samples), which are likely to better reflect the mantle source composition, have positive εSr (100 Ma) values (+8.1 to +14.5) and negative to slightly positive εNd (100 Ma) values (−1.6 to +2.5) consistent with derivation from an enriched mantle source. These eleven samples may be divided into two groups either characterized by higher LILE/HFSE ratios, less radiogenic εSr (100 Ma) values and more radiogenic εNd (100 Ma) values, or characterized by relatively lower LILE/HFSE ratios, more radiogenic εSr (100 Ma) values and less radiogenic εNd (100 Ma) values suggesting differences in the mantle source. The results of this study are consistent with the melting of a variably metasomatized sub-arc mantle source during a transition from a wrench to a transtensional tectonic setting, but are inconsistent with a mantle plume origin.
Geology, 1991
The Little Rincon thrust fault is a mylonitic shear zone that juxtaposes Middle Proterozoic Conti... more The Little Rincon thrust fault is a mylonitic shear zone that juxtaposes Middle Proterozoic Continental Granodiorite over metasedimentary rocks of Proterozoic and early Paleozoic age. This fault is structurally beneath the San Pedro detachment fault and associated ductile deformational fabrics, which formed during early Oligocene to early Miocene time. A syntectonic leucogranite sill within the Little Rincon shear zone yields a U-Pb concordia-intercept age of 66 ±10 Ma for zircon and a concordant age of 51 ±2 Ma for fractions composed of monazite and xenotime. This demonstrates that compressional deformation in the Catalina and Rincon mountains is generally coeval with Laramide thrust faults that extend at least from southeastern California to southeastern Arizona. A peraluminous granite pluton that truncates the shear zone but displays extension-related fabrics yields a lower-intercept age of 24 ±12 Ma for zircon and an age of 30 ±6 Ma for monazite. This indicates that some peraluminous plutons in the region were emplaced during regional crustal extension.
Elements, 2011
Pristine peritectic garnet (pink) and minor cordierite (pale green, labelled Crd) in leucosome in... more Pristine peritectic garnet (pink) and minor cordierite (pale green, labelled Crd) in leucosome in stromatic metatexite migmatite from the Fosdick migmatite-granite complex, West Antarctica. See Figure 1 in Sawyer et al. (2011 this issue); for more details see Korhonen et al. (2010).
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Papers by Christine Siddoway