Burial diagenesis: Out of sight, out of mind!
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SEPM Special Publication↗
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Influence of climate on deep-water clastic sedimentation: application of a modern model, Peru-Chile Trough, to an ancient system, Ouachita Trough
Traditionally, an abrupt and massive influx of siliciclastic sediments into an area of deposition has been attributed to tectonic uplift without consideration of the influence of climate or climatic change on rates of weathering, erosion, transportation, and deposition. With few exceptions, fluvial sediment transport is minimal in both extremely arid climates and in perhumid (everwet) climates. Maximum sediment transport occurs in climates characterized by strongly seasonal rainfall, where the effect of vegetation on erosion is minimal. The Peru–Chile trench and Andes Mountain system (P–CT/AMS) of the eastern Pacific Ocean clearly illustrates the effects of climate on rates of weathering, erosion, transport, and deep-sea sedimentation. Terrigenous sediment is virtually absent in the arid belt north of lat. 30° S in the P–CT, but in the belt of seasonal rainfall south of lat. 30° S terrigenous sediment is abundant. Spatial variations in the amount and seasonality of annual precipitation are now generally accepted as the cause for this difference. The spatial variation in sediment supply to the P–CT appears to be an excellent modern analogue for the temporal variation in sediment supply to certain ancient systems, such as the Ouachita Trough in the southern United States. By comparison, during the Ordovician through the early Mississippian, sediment was deposited at very slow rates as the Ouachita Trough moved northward through the southern hemisphere dry belt (lat. 10° S to lat. 30° S). The deposystem approached the tropical humid zone during the Mississippian, coincident with increased coarse clastic sedimentation. By the Middle Pennsylvanian (Atokan), the provenance area and the deposystem moved well into the tropical humid zone, and as much as 8,500 m of mineralogically mature (but texturally immature) quartz sand was introduced and deposited. This increase in clastic sediment deposition traditionally has been attributed solely to tectonic activity. However, we contend that the principal control on the introduction of abundant terrigenous sediment was the movement of the deposystem from an arid or semiarid climate into a seasonally wetter climatic regime. The physical and mineralogical maturity of the quartz sand is the result of tropical weathering in provenance areas.
Climate Controls on Stratigraphy: SEPM Special Pub↗
A modern analogue for tectonic, eustatic, and climatic processes in cratonic basins: Gulf of Carpentaria, northern Australia
The Gulf of Carpentaria is a tropical, silled epicontinental sea and may be a modern analogue for ancient cratonic basins. For the purpose of this study, the Gulf of Carpentaria is compared to Pennsylvanian cratonic basins of the United States. During the Pennsylvanian, the North American continent moved from the Southern Hemisphere, through the Equator, into the Northern Hemisphere. Today, the Gulf of Carpentaria–New Guinea region is a few degrees south of the Equator and is moving towards it. During the Pennsylvanian, the world was subjected to major glaciations and associated sea-level changes. The island of New Guinea and the Gulf of Carpentaria have undergone similar processes during the Quaternary. A reconnaissance seismic survey of the gulf conducted by the USGS and the Australian National University (ANU), combined with oil-exploration well data, provided the first step in a systematic evaluation of a modern tropical epicontinental system. During the Cenozoic, the region was dominated by terrestrial sedimentation in a temperate climate. At the same time, carbonates were being deposited on the northern shelf edge of the Australian Plate. During the Miocene, carbonate deposition expanded southward into the gulf region. Then in the Late Miocene, carbonate sedimentation was replaced by terrigenous clastics derived from the developing Central Range of the island of New Guinea, which developed a wetter climate while moving northwards into the tropics. At least 14 basin-wide transgressive–regressive cycles are identified by channels that were eroded under subaerial conditions since about the Miocene. Comparison of the modern Gulf of Carpentaria sequences with those of the Pennsylvanian reveals many similarities.
Gulf Of Carpentaria↗
Duststones on Mars: Source, transport, deposition and erosion
Dust is an abundant material on Mars, and there is strong evidence that it is a contributor to the rock record as “duststone,” analogous in many ways to loess on Earth. Although a common suite of dust formation mechanisms has operated on the two planets, fundamental differences in environments and geologic histories have resulted in vastly different weighting functions, causing distinct depositional styles and erosional mechanisms. On Earth, dust is derived predominantly from glacial grinding and, in nonglacial environments, by other processes, such as volcanism, eolian abrasion, and fluvial comminution. Hydrological and biological processes convert dust accumulations to loess deposits. Active hydrology also acts to clean dust from the atmosphere and convert loess into soil or erode it entirely. On Mars, glacial production of dust has been minor, with most fine particles probably produced from ancient volcanic, impact, and fluvial processes. Dust is deposited under arid conditions in which aggregate growth and cementation are the stabilizing agents. Thick accumulations result in duststone.
SEPM Special Publication↗
Origin of Franciscan melanges in Northern California
In northern California, chaotic Franciscan melange occurs beneath the overlying ophiolite and Great Valley Sequence. Identical melanges occur to the west, separating well-bedded, coherent Franciscan units that differ markedly in age. Detailed studies in several places indicate that these melanges mark the boundaries of imbricate thrust sheets, and they appear to occur at several discrete structural horizons. The melange comprises blocks of graywacke, greenstone, chert, serpentinite, and isolated so-called knockers of high-grade blueschist and eclogite set in a matrix of sheared and quartz-veined mudstone and minor sandstone. Except for the blocks of high-grade schist, these rocks are similar to, but more deformed than, the orderly sedimentary, volcanic, and other rocks that occur immediately above the Coast Range thrust at the base of the Great Valley Sequence. Unlike the other Franciscan units, the melanges contain relatively abundant fossils, mainly Buchia, radiolarians, and dinoflagellates. Significantly, all of these fossils are of Tithonian to Valanginian age. We suggest, on the basis of similarity of lithology and fossil content, that the matrix of the melanges represents a distal, or seaward, portion of the basal sediments of the Great Valley Sequence and that the abundant greenstone, chert, and serpentinite found as tectonic blocks within the melanges were derived from the underlying oceanic crust and upper mantle. Formation of the melanges must be related to multiple subduction of separate plates, the melange beinggenerated repeatedly from the ultramafic-mafic-chert and Buchia-hearing shale and minor graywacke sequence that constitutes the oldest rocks of the Coast Ranges. This process of subduction probably began in the Early Cretaceous and continued into the Tertiary, as Eocene fossils have been found recently in deformed Franciscan (coastal belt) rocks structurally below the melanges. The tectonic blocks of high-grade blueschist and eclogite were formed during an earlier period of subduction, then embedded in serpentinite and carried westward by flow in the upper mantle. During subsequent subduction, the serpentinite and embedded blocks of schist were tectonically mixed with the overlying volcanic rocks, chert, graywacke, and fossiliferous shale.
SEPM Special Publication↗
Linear island and seamount chains, aseismic ridges and intraplate volcanism: Results from DSDP
The Deep Sea Drilling Project drilled a substantial number of sites that bear on the origin of linear island and seamount chains, aseismic ridges and other more regional expressions of intraplate volcanism. Drilling in the Emperor Seamounts during Leg 55 was particularly successful. Results from this leg include: 1) the volcanoes of the Hawaiian-Emperor chain continue to increase in age away from Kilauea as predicted. 2) Suiko Seamount formed at a paleolatitide of 26.9±3.5°N, 7° north of present-day Hawaii, but far south of its present latitude of 44.8°N. 3) the volcanic rock types recovered include hawaiite, mugearite, alkalic basalt and tholeiitic basalt in the sequence and relative volume expected for Hawaiian volcanoes. 4) the tholeiitic and alkalic basalts recovered are geochemically similar to those in the Hawaiian Islands, only the ratio of 87 Sr/ 86 Sr appears to change through time. All the lavas appear to be derived from a source that has small-scale heterogeneities, but is homogeneous on a large scale. 4) The Emperor Seamounts were once volcanic islands that underwent subaerial and shallow marine erosion, and deposition of shallow-water biogenic carbonate sediments that capped all or most of each volcano. Drilling in other regions has yielded less conclusive results. For example, it is uncertain if the Line Islands are an age progressive chain (hot-spot trace) or result from some other type of intraplate volcanism. The mid-Pacific Mountains also show evidence of originating from a regional episode of volcanism in the mid-Cretaceous. Drilling in the Nauru Basin encountered a voluminous mid-Cretaceous volcanic flow-sill complex that overlies Jurassic magnetic anomalies, yet is composed of depleted tholeiite. In the Indian Ocean, drilling on the Ninety-East Ridge established that it 1) is volcanic in origin; 2) is older to the north; 3) formed in shallow water, and 4) formed further south and has moved northward. It appears that the Ninety-East Ridge, like the Hawaiian-Emperor chain, is a hot spot trace. In the Atlantic Ocean, drilling on the Iceland-Faeroe Ridge and the Rio Grande Rise-Walvis Ridge suggests that all these aseismic ridges are hot spot traces generated by the Iceland and Tristan de Cunha hot-spots.
SEPM Special Publication↗
Holocene coastal development on the Florida peninsula
The Florida peninsula contains five distinct coastal sections, each resulting from its own spectrum of coastal processes and sediment availability during a slowly rising, late Holocene sea level. The east coast barrier system is wave-dominated and has a large cuspate foreland (Cape Canaveral) near its middle. The Florida Keys and reef tract represent the only coastal carbonate system in the continental United States. An open-marine mangrove coast characterizes the low-energy, tide-dominated southwest part of the State. The central Gulf barrier system displays a mixed-energy morphology in a microtidal, low-energy setting. The open-coast marsh system of the Big Bend area that is north of the barrier system is also tide dominated, and is developed on a sediment-starved carbonate platform. The oldest preserved coastal Holocene section is the Florida Keys area where, at about 6 to 8 ka, sequences accumulated during the Holocene. Most of the remainder of the peninsular coast is characterized by terrigenous sequences less than 3 ka. The younger sequences accumulated almost exclusively from reworking of older strata without benefit of additional sediment supply from land.
Florida↗
Stratigraphy of the Mississippi-Alabama shelf and the Mobile River incised-valley system
The Mobile River incised-valley system located in the northern Gulf of Mexico occupies an area from southern Alabama through Mobile Bay to the outer Mississippi-Alabama continental shelf. During the Wisconsinan regression, this incised-valley system was fluvially eroded and extended across the exposed shelf to a shelf-margin delta complex. The last postglacial transgression drowned the entrenched alluvial valleys and reworked the alluvial fill and estuarine deposits to form shoals on the middle shelf. As the postglacial transgression slowed. Mobile Bay was formed. Mobile Bay is a large estuarine system protected by barrier islands. This paper documents the late Quaternary history of the Mobile River incised valley and fill. Mobile Bay is a large (> 1000 km 2 ) microtidal estuary in southern Alabama that receives drainage through the Mobile River system. The Mobile River catchment is the fourth largest in the United States and terminates at the bayhead delta of Mobile Bay. The bay is a classic example of a wave-dominated, drowned, fluvial incised valley. During the middle-late Wisconsinan, glacial maximum, relative sea level was approximately 120 m lower than present. The Mobile River incised valley was a conduit for drainage from the catchment to the shelf margin. The sediment carried by the fluvial system during this lowstand passed through the Mobile River incised valley, across the exposed shelf and was deposited on the shelf margin as deltaic lobes. Rapid sea-level rise forced coastal-plain shorelines landward across the present mid-continental shelf. Transgression of the estuary mouth left a series of estuary-mouth-bar deposits that were reworked and overlie alluvial fill. These sand-rich deposits were submerged by the continuing transgression and reworked to form shoals As the Holocene sea-level rise slowed, the Mobile River incised valley became an estuarine depocenter. In the present alluvial valley, lowstand deposits are overlain by estuarine sediments deposited during the initial flooding of the valley and subsequent formation of Mobile Bay. During the present highstand, longshore sediment transport formed a spit across much of the bay mouth, creating a restricted estuary into which a bayhead delta has prograded. Late Holocene deposits in Mobile Bay consists predominantly or lagoonal sediments with bayhead-delta deposits encroaching into the northern end of the bay. The Holocene incised-valley fill (estuarine facies) underlying Mobile Buy fit well into the conceptual facies model of a microtidal wave-dominated estuary. The model does not fit as well, however, with the rapidly transgressed shelf portion of the incised valley. The down dip section does not contain a clearly identifiable (from seismic profiles) estuarine facies; the valley fill is primarily fluvial and is overlain by marine shoals. In the Mobile River incised valley, the distal portion of the valley was rapidly drowned, allowing the thin estuarine facies to be reworked. The proximal portion was drowned more slowly, leaving the estuarine facies intact. Thus, the single incised valley contains two very different types of fill.
Alabama↗
Comparative geochemical and mineralogical studies of two cyclic transgressive pelagic limestone units, cretaceous Western Interior Basin, U.S.
Pelagic limestone units were deposited in the North American Western Interior seaway during two major Cretaceous transgressive episodes. The Bridge Creek Limestone Member of the Greenhorn Formation, deposited during the Late Cenomanian-Early Turonian transgression, and the Smoky Hill Member of the Niobrara Formation, deposited during the overall Early Coniacian-Early Campanian transgression, are both enriched in organic-carbon and exhibit smallscale carbonate cycles representing periodicities in the range 20 to 40 ky. The distinct periodicity and overall unusual depositional milieu of both units are reflected in their sedimentary structures, mineralogy, and geochemistry. The Bridge Creek Limestone at Pueblo, Colorado, averages 78% CaCO 3 and 1.75% organic carbon with ranges of 42-96% and 0.06-6.97%, respectively, across small-scale cycles. High concentrations of Al, Fe, Mg, K, Ti, Na, Cr, Ni, V; higher Sr/Ca and lower Si/Al ratios; and lighter δ 18 0 in CaCO 3 in dark-colored clay-rich beds all suggest periodic influx of terrestrial clay minerals during times of peak fresh water runoff from uplifted highlands to the west. Higher Sr/Ca ratios in marlstone beds than in limestone beds suggest that the marlstone beds have undergone less diagenetic removal of Sr. Higher concentrations of organic carbon, hydrogen, and sulfur, and preservation of some lamination in the clay-rich beds also suggest that the times of enhanced runoff may have induced stable salinity stratification in the water column, which led to gradual depletion of dissolved oxygen in the bottom waters and enhanced preservation of organic carbon in sediments. The geochemistry also suggests that a significant change in sedimentation occurred at the Cenomanian-Turonian boundary. The geochemical characteristics of the Niobrara Formation near Fort collins, Colorado, are very similar to those of the Bridge Creek Limestone at Pueblo, suggesting similar depositional conditions and source of clastic materials. However, the small scale cycles are present but more subdued in the Niobrara Formation than in the Bridge Creek Limestone, and the Niobrara Formation in the Fort Collins area has not been as altered by diagenesis.
SEPM Special Publication↗
Mississippian sedimentary facies patterns in east-central California and implications for development of the Permian last chance thrust
Mississippian sedimentary facies belts in east-central California, occurring primarily in the autochthon (lower plate) of the Last Chance Thrust, are consistently oriented in a northeast–southwest direction. The boundary of one belt is marked by the depositional limit of the Osagean to Meramecian Santa Rosa Hills Limestone; a second belt farther to the northwest is bordered by the erosional truncation of the Kinderhookian to Osagean Tin Mountain Limestone. Two additional facies belts, both in the Meramecian to Chesterian Kearsarge Formation, also are present in the area; one near Jackass Flats is marked by the presence of limestone and quartzite olistoliths, and...
SEPM Special Publication↗
Evolution of the last chance thrust concept
The concept of the Permian Last Chance Thrust has passed through many stages. Here we point out which critical observations have led to changes in the way this important feature has been interpreted.
SEPM Special Publication↗
Bottom-sediment chemistry in Devil's Lake, northeast North Dakota
Devils Lake is a 200 km 2 terminal lake that contains sodium sulfate type water. Dissolved solids concentrations range from about 3,500 mg/L to 10,000 mg/L depending on location To investigate geochemical processes in the bottom sediments of Devils Lake, sediment cores were collected at two sites in the western half of the lake during a period of bottom water oxygen depletion. The upper 10 cm of the sediments consist of about 60 weight percent silicates (quartz, feldspar, and clays) 35 weight percent carbonates and 5 weight percent organic material. At depths between 1 and 3 cm in the sediments bacterial sulfate reduction and associated degradation of organic material cause minima in sulfate concentrations and δ 13 C values of dissolved inorganic carbon and maxima in alkalinity, ammonia, phosphate, and sulfide concentrations and δ 34 S values of dissolved sulfate. Downward increases of sodium, magnesium, potassium, and calcium concentrations result from upward diffusion of ions from saline pore water and dissolving sulfate minerals below 30 cm depth in the sediments. High magnesium calcite 8 mole percent MgCO 3 is the most abundant carbonate at the sediment surface. With increasing depth abundances of high magnesium carbonate decrease and abundances of low magnesium calcite aragonite and dolomite increase. Carbon isotope compositions of bulk carbonates range from δ 13 C = -0.7 to +0.5%. These values are close to equilibrium with dissolved inorganic carbon in lake water ( δ 13 C = -2%) but far from equilibrium with dissolved inorganic carbon in pore water ( δ 13 C = -16.3- -10/0%). Disequilibrium between pore water and carbonates suggests that the carbonates did not recrystallize substantially in the presence of pore water. Therefore the change of carbonate mineral proportions with depth in the sediments is due mainly to temporal changes in the proportions of endogenic, detrital, and biologic carbonates that were deposited on the lake bottom rather than postdepositional carbonate diagenesis.
North Dakota↗