Volcanism: Eruptions and Extinctions

Fossils from southern China provide evidence for a mass extinction during middle Permian time, 260 million years ago. The close association of this event with an outpouring of lava, initially into the sea, indicates that explosive volcanism may have been the cause

Onlap, Offlap, and the Origin of Unconformity-Bounded Depositional Sequences

Unconformity-bounded depositional sequences represent the fundamental building blocks of sedimentary successions. They are typically characterized by onlap at the base and by offlap at the top, and they tend to be markedly asymmetrical, with onlap accounting for a larger part of any cycle of sedimentation than offlap. Offlap cannot be attributed solely to erosional truncation, but instead implies that sequence boundaries develop over a finite interval of time. Depositional sequences are commonly associated with a cyclic arrangement of facies, but transgressive-regressive cycles are out of phase with respect to sequence boundaries, which in down-dip locations are both overlain and underlain by progradational deposits, and hence form during times of regression of the shoreline. These observations are used to develop some ideas about the origin of unconformity-bounded sequences, with reference to the inter-related roles of changes in depositional base level and sediment supply. In particular, it is shown than onlap and offlap are due to lateral migration of a “line of critical bypassing”, defined so as to incorporate the effects of sediment loading and compaction as well as the rate of change of elevation with respect to sea level. Downward shifts in onlap may be achieved by either an increase in the rate of eustatic fall or a decrease in the rate of tectonic subsidence, and it is premature to assume that eustatic and tectonic controls on sea level may be distinguished solely on the basis of the frequency of depositional cyclicity. Small shifts in the position of onlap can also be produced by changes in sediment supply, and more attention needs to be paid to the influence of sediment supply in the development of minor boundaries. Unconformities related to eustatic fluctuations are thought to correspond approximately to times of relatively rapid sea-level fall (inflection points), but questions remain about the existence of possible leads and lags of up to View the MathML source cycle, and hence about the degree to which sequence boundaries of eustatic origin may vary in age both within a given basin and from one basin to another

Pre-Pleistocene Glaciation on Earth: Implications for Climatic History of Mars

The history of ice ages on Earth, extending back more than 2 Ga (109 years), has been established by the recognition in strata of many ages of the assemblage of erosional features and deposits generated by glacial activity. The most useful indicators are widespread diamictite, striated and faceted clasts, polished and striated pavements, and laminites containing “dropstones” inferred to be ice-rafted. The chronology of ancient glaciation is limited by our ability to recognize the results of glacial activity in an incomplete geologic record and by the age resolution attainable with available dating methods. Ice ages are currently known from the Early Proterozoic (2.5-2.1 Ga ago), several intervals in the Middle to Late Proterozoic (1.0-0.57 Ga), the Late Ordovician to Late Silurian (440-415 Ma, or 106 years), possibly latest Devonian (360-345 Ma), Permo-Carboniferous (335-245 Ma), and late Cenozoic (26 Ma ago to the present). However, the timing of pre-Phanerozoic ice ages, occurring before 0.57 Ga ago, is known only approximately, and glacial events would have been shorter than is suggested by the bracketing ages. A generally warm climate seems to have prevailed for the remainder of Earth history, although Mesozoic-Cenozoic seismic stratigraphic evidence suggests that a small ice sheet may have been a persistent feature near the south pole throughout the last 500 Ma. Terrestrial climatic fluctuations occur on several time scales. The occurrence of ice ages on a time scale of 108−109 years, and possibly higher-frequency fluctuations (106−107 years), appears to be controlled by changes in solar output and atmospheric composition, and by the rearrangement of continents and oceans as a result of the motion of lithospheric plates. Climatic changes on a time scale of 104−105 years are driven by perturbations of the Earth's orbital parameters. The principal evidence suggesting that significant climate changes have occured on Mars consists of widespread dry channels (3.5-0.5 Ga?) and of geologically young dust-ice layered terrains in the polar regions (less than 10 Ma?). By analogy with the Earth, polar layered terrains have probably existed episodically on Mars for a considerable time although there is little direct evidence for this. Factors affecting a potential sedimentary record are the degree to which ice sublimed or melted, the effectiveness of eolian destruction of layering, and, in contrast to the present polar terrains, the tendency for ice to flow and slide over its substrate. As on Earth, climate changes on Mars have probably been induced by the long-term evolution of the atmosphere, by changes in solar luminosity, and by variations in orbital parameters. Major tectonic events may have affected Martian climate through changes in planetary obliquity

Upper Proterozoic and Lower Cambrian Rocks of the Sheeprock Mountains, Utah: Regional Correlation and Significance

Apparently conformable upper Proterozoic and Lower Cambrian miogeoclinal rocks in the Sheeprock Mountains, Utah, attain a maximum thickness of at least 7,200 m. The sequence begins at the base with phyllite, quartzite, glaciomarine diamictite, and shale deposited near the northern edge of a subsiding basin. These rocks are assigned to the Sheeprock Group (2,700–4,300 m). Overlying quartzitic rocks (1,950–4,000 m) are correlated with specific formations of the Brigham Group (Huntsville sequence). Revision of earlier accounts of the stratigraphy in the Sheeprock Mountains is suggested by the recognition of low-angle faults that attenuate the stratigraphic section. Stratigraphic relations in the Sheeprock Mountains bear on regional correlation. The probable presence in the Deep Creek Range of two diamictite units separated by quartzite is reaffirmed. This sequence is grossly similar to that of the Sheeprock Mountains. It is suggested that the Caddy Canyon Quartzite (Brigham Group) inter-fingers to the south and west of the Sheeprock area with siltstone, shale, and some limestone. Possibly, no rocks exposed in the San Francisco Mountains and Canyon Range are older than the Caddy Canyon Quartzite. The McCoy Creek Group of western Utah and eastern Nevada is probably for the most part equivalent to the Caddy Canyon Quartzite. The Osceola Argillite (unit G, McCoy Creek Group) may be equivalent to the Inkom Formation, and it perhaps records a marine transgression that temporarily reduced the clastic supply. The correlation of the Mutual Formation of the platform sequence in the Wasatch Range to a lithologically similar unit in the miogeocline to the west remains the simplest interpretation, although the platform Mutual may be older than the unit of the same name in the miogeocline

Structural Geology of the Southern Sheeprock Mountains, Utah: Regional Significance

The Sheeprock Mountains are part of a horst of Proterozoic, Paleozoic and Cenozoic sedimentary and igneous rocks located in the transitional region between the Cordilleran fold-thrust belt and the hinterland in the Basin-Range province of west-central Utah. Prominent structural elements in the Sheeprock Mountains are the Sheeprock Thrust, juxtaposing Proterozoic rocks above Paleozoic ones with a stratigraphic separation exceeding 10 km; the Pole Canyon Thrust, thought to be an upper plate imbrication of the Sheeprock Thrust; the Pole Canyon Anticline, a recumbent fold vergent to the northeast and cut by the Pole Canyon Thrust; the east-northeast-striking Indian Springs (tear) Fault; and two low-angle normal faults (the Harker and Lion Hill Faults) which together account for stratigraphic omission of several kilometres. The Pole Canyon Anticline is thought to have developed in the late Mesozoic during propagation of the thrusts parallel to the Indian Springs Fault, and this transport direction is corroborated by minor structures. Fault geometry suggests that the Harker and Lion Hill Faults are younger than the thrusts and probably of late Cenozoic age, although some mid-Cenozoic or even earlier displacement cannot be entirely ruled out. My [The author's] preferred interpretation of the structural history of the Sheep rock Mountains is consistent with minimal regional extension before the mid-Cenozoic and with the view that crustal shortening in the fold-thrust belt is for the most part unrelated to hinterland extension

Pre-Mesozoic Palinspastic Reconstruction of the Eastern Great Basin (Western United States)

The Great Basin of the western United States has proven important for studies of Proterozoic and Paleozoic geology [2500 to 245 million years ago (Ma)] and has been central to the development of ideas about the mechanics of crustal shortening and extension. An understanding of the deformational history of this region during Mesozoic and Cenozoic time (245 Ma to the present) is required for palinspastic reconstruction of now isolated exposures of older geology in order to place these in an appropriate regional geographic context. Considerable advances in unraveling both the crustal shortening that took place during Mesozoic to early Cenozoic time (especially from about 150 to 50 Ma) and the extension of the past 37 million years have shown that earlier reconstructions need to be revised significantly. A new reconstruction is developed for rocks of middle Proterozoic to Early Cambrian age based on evidence that total shortening by generally east-vergent thrusts and folds was at least 104 to 135 kilometers and that the Great Basin as a whole accommodated ∼250 kilometers of extension in the direction 287° ± 12° between the Colorado Plateau and the Sierra Nevada. Extension is assumed to be equivalent at all latitudes because available paleomagnetic evidence suggests that the Sierra Nevada experienced little or no rotation with respect to the extension direction since the late Mesozoic. An estimate of the uncertainty in the amount of extension obtained from geological and paleomagnetic uncertainties increases northward from ±56 kilometers at 36°30N to -87+108 kilometers at 40°N. On the basis of the reconstruction, the original width of the preserved part of the late Proterozoic and Early Cambrian basin was about 150 to 300 kilometers, about 60 percent of the present width, and the basin was oriented slightly more north-south with respect to present-day coordinates

Tectonic Subsidence of the Early Paleozoic Passive Continental Margin in Eastern California and Southern Nevada

Quantitative analysis of tectonic subsidence in Cambrian and Ordovician platform carbonates and associated strata exposed in the Spring Mountains (Nevada) and the Nopah, Funeral, and Inyo Ranges (California) indicates that subsidence associated with this segment of the early Paleozoic passive continental margin is exponential in form, consistent with thermal contraction of the lithosphere following extension. As in other parts of the North American Cordillera, continental separation in the southern Great Basin appears to have taken place between 590 and 545 Ma. These results are not sensitive to uncertainties in stratigraphic thickness, biostratigraphic age control, or paleobathymetry. Uncertainties in the Cambrian time scale lead to predictable variations in the inferred time of onset of thermal subsidence, but they have no effect on the inferred stratigraphic position of the rift to post-rift transition. A younger age for the base of the Middle Cambrian results in a younger inferred age of onset of thermal subsidence accompanied by greater rates of subsidence during the Cambrian, whereas a significantly older estimate of the onset of thermal subsidence can be obtained only if the base of the Middle Cambrian is substantially older than 540 Ma, a possibility that is inconsistent with available data. Results of the subsidence analysis are particularly significant because this is one of the few regions along the length of the North American Cordillera where they can be compared directly to the geologic evidence for syn-rift and post-rift deposition. Basement-involved faulting associated with the Amargosa basin ("aulacogen") ceased during deposition of the Noonday Dolomite, which is thought to be older than 700-680 Ma on the basis of stromatolites of late Riphean affinity. The overlying Johnnie Formation contains supposed Vendian stromatolites (younger than 700-680 Ma). If it is assumed that our results indicate the timing of the final rift to post-rift transition, then either the ages inferred from stromatolites are incorrect or the lithosphere was thinned regionally after deposition of the Noonday. The latter possibility is supported by limited geologic evidence for extension in latest Proterozoic and Early Cambrian time. The lack of appreciable physical evidence for crustal extension after deposition of the Noonday, however, may imply that (1) a uniform extension model for lithospheric thinning is inappropriate for this part of the margin or that (2) some or all of the localities studied are continentward of the hinge zone, and that the observed subsidence is exaggerated by flexural loading in a deeper basin to the west

A New Hypothesis for the Amount and Distribution of Dextral Displacement along the Fish Lake Valley–Northern Death Valley–Furnace Creek Fault Zone, California-Nevada

The Fish Lake Valley–northern Death Valley–Furnace Creek fault zone, a ~250 km long, predominantly right-lateral structure in California and Nevada, is a key element in tectonic reconstructions of the Death Valley area, Eastern California Shear Zone and Walker Lane, and central Basin and Range Province. Total displacement on the fault zone is contested, however, with estimates ranging from ~30 to ~63 km or more. Here we present a new synthesis of available constraints. Preextensional thrust faults, folds, and igneous rocks indicate that offset reaches a maximum of ~50 km. Neogene rocks constrain its partitioning over time. Most offset is interpreted as ≤ ~13–10 Ma, accruing at ~3–5 mm/yr in the middle of the fault zone and more slowly toward the tips. The offset markers imply ~68 ± 14 km of translation between the Cottonwood Mountains and Resting Spring–Nopah Range (~60 ± 14 km since ~15 Ma) through a combination of strike slip and crustal extension. This suggests that a previous interpretation of ~104 ± 7 km, based on the middle Miocene Eagle Mountain Formation, is an overestimate by ~50%. Our results also help to mitigate a discrepancy in the ~12–0 Ma strain budget for the Eastern California Shear Zone. Displacement has previously been estimated at ~100 ± 10 km and ~67 ± 6 km for the Basin and Range and Mojave portions of the shear zone, respectively. Our new estimate of ~74 ± 17 km for the Basin and Range is within the uncertainty of the Mojave estimate

Is the Sevier Desert Reflection of West-Central Utah a Normal Fault?

A prominent west-dipping reflection that can be traced in seismic-reflection profiles over an area of 7000 km2 beneath the Sevier Desert basin of west-central Utah is generally referred to as the Sevier Desert detachment and is widely regarded as one of the best examples of an upper-crustal low-angle normal fault. The absence of evidence for fault-related deformation in drill cuttings and core from two industry boreholes that intersect this feature casts doubt on the fault interpretation. The existing interpretation is based mainly on the observation that high-angle normal faulting is restricted largely to Tertiary sedimentary and volcanic rocks above the reflection. An alternative explanation is that the high-angle faults are related to the withdrawal of early deposited lacustrine salt, which even today is as much as 2 km thick. Reevaluation of the seismic data suggests that the Sevier Desert reflection consists of two spatially and genetically distinct segments: a shallow segment here interpreted as an unconformity between Paleozoic and Tertiary strata and a fortuitously aligned deeper segment that is traceable to mid- and lower-crustal levels and that appears to represent a thrust fault related to the Cretaceous Sevier orogeny

Geological Time Conventions and Symbols

All science involves conventions. Although subordinate to the task of figuring out how the natural world functions, such conventions are necessary for clear communication, and because they are a matter of choice rather than discovery, they ought to reflect the diverse preferences and needs of the communities for which they are intended. A short article published recently in both Pure and Applied Chemistry and Episodes (Holden et al., 2011a, 2011b) sets out to rationalize the definition and symbols for units of time for use in nuclear chemistry and the earth and planetary sciences. Given that the authors are members of a task group established jointly by the International Union of Geological Sciences (IUGS) and the International Union of Pure and Applied Chemistry (IUPAC), and that publication was approved by both bodies, one might reasonably assume that the recommendations reflect a workable consensus. Regrettably, they don't. They will be widely ignored in North America. How could the peer review system fail so badly in this case? What needs to be done