Red Bed Sequence Research Articles

Intercontinental correlation by sea-level events in the Early Silurian of North America and China (Yangtze Platform)

Research Article| November 01, 1985 Intercontinental correlation by sea-level events in the Early Silurian of North America and China (Yangtze Platform) MARKES E. JOHNSON; MARKES E. JOHNSON 1Department of Geology, Williams College, Williamstown, Massachusetts 01267 Search for other works by this author on: GSW Google Scholar RONG JIA-YU; RONG JIA-YU 2Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing, People's Republic of China Search for other works by this author on: GSW Google Scholar YANG XUE-CHANG YANG XUE-CHANG 2Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing, People's Republic of China Search for other works by this author on: GSW Google Scholar Author and Article Information MARKES E. JOHNSON 1Department of Geology, Williams College, Williamstown, Massachusetts 01267 RONG JIA-YU 2Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing, People's Republic of China YANG XUE-CHANG 2Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing, People's Republic of China Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1985) 96 (11): 1384–1397. https://doi.org/10.1130/0016-7606(1985)96<1384:ICBSEI>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation MARKES E. JOHNSON, RONG JIA-YU, YANG XUE-CHANG; Intercontinental correlation by sea-level events in the Early Silurian of North America and China (Yangtze Platform). GSA Bulletin 1985;; 96 (11): 1384–1397. doi: https://doi.org/10.1130/0016-7606(1985)96<1384:ICBSEI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Potential for eustatic sea-level changes on continental platforms during Early Silurian (Llandovery) time was highly propitious due to the double effects of lingering glaciation in Gondwana and the rapid rate of sea-floor spreading which assisted the general dispersion of many era tonic realms through middle Paleozoic oceans. Following the initial mass melting of the Late Ordovician/Early Silurian glaciers in North Africa and Arabia, four smaller cycles of sea-level fluctuation were recorded by recurrent, shelly communities in a predominantly carbonate setting on the North American Platform. Peaks in sea-level rise occurred in late Rhuddanian or early Idwian (A4–B1 positions), early Fronian (C1 position), early Telychian (C4 position), and late Telychian (C6 position) times. Although absolute time control is poor, the periodicity of these events may have been every 2.5 m.y.In the more submerged regions of the North American Platform, such as Iowa and Anticosti Island, diverse faunal associations distinguished by the presence of stricklandiid brachiopods were especially productive at peak times of transgression. Co-occurrence of the calcareous green alga Cyclocrinites with certain of these faunas indicates that maximum water depth was still within the photic zone. At the other extreme, low stands in sea level are identified by diminished faunal diversity, as well as a combination of typical sedimentary structures and evaporite indicators.Located on a well-defined Precambrian craton, the Yangtze Platform is now a large region in central and southwestern China. During successive stages of the Early Silurian, a thick sequence of graptolitic shales, shelly carbonates, and red beds were deposited on the platform. Of the four sea-level peaks defined in North America, three are recognized in the Yangtze Region. There is very good evidence for a latest Llandovery or earliest Wenlock transgression, based on the extensive distribution of the Salopinella-Coronocephalus-Sichuanoceras fauna. Another clear and well-dated sea-level peak involves the occurrence of faunas with characteristic stricklandiid and pentamerid brachiopods at a level equivalent to the lower Telychian (C4 position). The initial Silurian transgression appears to have reached its peak on the Yangtze Platform primarily during early Idwian time (B1 position), but this is based less surely on changing patterns in graptolite diversity. At present, there is no compelling evidence for a widespread, early Fronian (C1 position) transgression in China. Successful matching of the other sea-level events, however, substantially refines intercontinental correlation for Lower Silurian strata in the Yangtze Region of China and the central regions of North America. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Exploration Potential of Paleozoic Rocks of Morocco: ABSTRACT

Paleozoic rocks of Morocco have some similarities with the producing Paleozoic sequences in Algeria. In Morocco, there is a basic division between the cratonal sequences of the Tidouf basin and continguous areas and the Paleozoic megabasin to the north and possibly to the west under the present continental shelf areas. The Paleozoic of the northern megabasin has the following positive exploration elements. (1) Both wrench and normal extensional tectonics have produced significant structures that may have been reactivated during 2 later orogenic events. (2) Reservoir quality, although poor in outcrop, can be significantly improved in the subsurface. Wells from the Bojad region of the Tadla basin encountered porosities up to 30% in Devonian clastic sequences. (3) No area ca be condemned on the basis of present published geochemical evidence. Burial depths are sufficient for mature hydrocarbons, and rocks with organic material are present in sediments ranging in age from Cambrian to Carboniferous. Paleozoic oil shows have been encountered and may actually serve as the source of hydrocarbon in the Essouira basin in a downfaulted Triassic red-bed sequence. (4) Quality of seismic data is good, even where Paleozoic rocks are onlapped by Mesozoic and Cenozoic sediments. Drilling for Paleozoic targets has been sparse, hence, few data are available to test both source and reservoir potential. Paleozoic rocks still need to be tested by industry and must be considered a frontier area. End_of_Article - Last_Page 252------------

Transitions in a Fluvial System--Abo Formation, Southeastern Nacimiento Mountains, New Mexico: ABSTRACT

The Abo Formation (Wolfcampian) in the southeastern Nacimiento Mountains, New Mexico, is an alluvial plain red-bed sequence which records the evolution of a fluvial system within the distal reaches of a large clastic wedge. Sediment was derived from the southern Uncompahgre uplift, with a paleoflow to the southwest. The Abo Formation consists of mudstone, feldspathic sandstone, and intraformational conglomerate. Three major lithofacies associations, representing distinct fluvial depositional settings, may be distinguished. The lowermost division (Unit A) (100+ m, or 330+ ft, thick) is mudstone dominant with isolated sandstone bodies showing cutbanks and lateral accretion surfaces. Ridge-and-swale features were noted on an exhumed point bar surface. The sandstones commonly show a progressive upward decrease in the scale of internal bedforms with a lower scour surface containing intraformational conglomerate. Trough cross-bedding is the dominant bedform in the sandstones with common planar-tabular and flat-bedded interbeds. The uppermost sandstones, characteristic of levee and crevasse-splay sands, are ripple cross-bedded with abundant bioturbation and some plant roots. The overbank mudstones contain numerous pedogenic horizons characterized by calcrete nodules (representative of all stages of evolution), vein networks, pseudo-anticlinal structures, slickensides, color mottling, and bioturbation. Vertebrate remains are found in the mudstones. The middle division (Unit B) (approximately 70 m, or 230 ft, thick) is characterized by thick multilateral channel sandstones and thinner mudstones. The sandstones are coarse grained and contain common intraformational conglomerate. Large-scale trough cross-stratification is the dominant bedform. Pedogenic horizons are present but less common than in unit A. The uppermost division (Unit C) (approximately 30 m, or 100 ft, thick) is entirely composed of reddish-orange, very fine-grained sandstone. Flat bedding is the dominant bedform. High aggradation is indicated by abundant dish structures in the sandstones and aggradational ripple cross-bedding. Mudstone is rare and preserved as thin laminae. End_Page 466------------------------------ The fluvial sequence from Unit A to Unit B is thought to represent the transition from a fine-grained to a coarse-grained meander-belt system. The transition to Unit C reflects a change to flash-flood, ephemeral-stream controlled sedimentation. Allocyclic phenomena are suggested as probably cause of these fluvial transitions. End_of_Article - Last_Page 467------------

Cedar Creek--A Significant Paleotectonic Feature of Williston Basin: ABSTRACT

Cedar Creek is the major anticlinal structure demarcating the south-west flank of the Williston basin. This pronounced fold developed through a geologic history of recurrent tectonic movements along a northwest-southeast striking fault zone. The four major periods of tectonism documentable in the Cedar Creek area from early Paleozoic through mid-Tertiary affected the local and regional distribution, erosion, and/or preservation, and, though moderately, the depositional facies of sedimentary strata since Ordovician time. Post-Silurian-Pre-Middle-Devonian: Uplift and fault movement accompanied north and east tilting of the main Cedar Creek block. Several hundreds of feet of Silurian strata were eroded prior to Middle Devonian time, and a karst plain developed on the Silurian surface. Middle and Upper Devonian sediments progressively onlapped and infilled the uplifted, northwest plunging element. Late Devonian-Pre-Mississippian: During latest Upper Devonian--possibly earliest Mississippian--pronounced fault movement occurred along the major fault zone. The Cedar Creek block was uplifted and tilted north and east; maximum displacement along the zone appears to have occurred in the vicinity of the Pine-Gas City-Glendive oil fields. Extensive erosion resulted in the near peneplanation of the structure and significant truncation of Upper Devonian strata. Continued paleostructural influence during Mississippian time is indicated by the distribution and facies of early and middle Mississippian rocks. Late Mississippian (Chester) through Triassic: In late Mississippian (Chester) and early Pennsylvanian time, the central and northern portion of the Cedar Creek area underwent gentle downwarping, and periods of subsidence occurred with relative down-to-the-east fault movement along parts of the ancestral master and subsidiary faults. Similar fault movement(s) and subsidence which continued to occur during the Permian and Triassic periods significantly influenced the deposition and preservation of these evaporite rich, red-bed sequences. Relative tectonic stability was attained by the Middle Jurassic and essentially maintained until post-Paleocene time. Post-Paleocene: The Cedar Creek block underwent its greatest magnitude of uplift during the post-Paleocene; younger Tertiary beds are not present in the area. This uplifting was accompanied by major flexuring, the extensive linear belt of asymmetric drape-folding generally aligned with the ancestral fault zones, and deep fault adjustment. Northwest plunge along the crestal portion was significantly increased. The entire area was subsequently uplifted during epeirogenic phases of the mid-Tertiary in the northern Rocky Mountain region and about 1,500 ft (460 m) of Paleocene and Upper Cretaceous strata eroded along the axis of the present structure. End_of_Article - Last_Page 1332------------

The architecture of an alluvial suite: rocks between the Townsend Tuff and Pickard Bay Tuff Beds (early Devonian), southwest Wales

The Townsend Tuff Bed and the Pickard Bay Tuff Bed are distinctive ash-fall tuffs traceable within a red-bed sequence over an area measuring about 35 km from west to east by 12 km from south to north. The succession between these markers is a mere 15-30 m thick, but contains six other tuffs which, though not themselves distinctive, can also be traced over the area by virtue of their position relative to the two main falls. The mudstone-dominated succession between the two main markers contains eight calcretes (pedogenic limestones), most of which extend over practically the whole of the area, and 16 generally local upward-fining or ‘coarse’ sequences composed of sandstones overlying an erosion surface and passing up into mudstones. The mudstones are massive and seldom laminated, apparently having been organically destratified. The coarser-grained upward-fining sequences involve cross-bedded, parallel-laminated and cross-laminated sandstones, whereas the finer-grained examples normally consist of interbedded mudstones and sharp-based cross-laminated sandstones. Intraformational conglomerates and stringers of mudstone clasts not infrequently accompany the coarser sandstones. Evidence of subaerial exposure is common towards the tops of both the coarser- and finer-grained sorts of upwardfining sequence. The careful tracing of the marker tuffs shows the upward-fining sequences to be of two main kinds. Examples of the least common type occur within localized and relatively deep depressions, probably incised valleys, which they fill to a height less than the depth of the depression. The more frequent variety is also localized but not obviously incised, having apparently grown up in harmony with the accumulation of mud in the area as a whole. The succession between the Townsend Tuff and Pickard Bay Tuff Beds lies in the transition between a clearly marine-influenced facies below and a clearly fluviatile facies above. It appears to record an extensive but comparatively featureless marginal mudflat influenced by both rivers and the sea. The development of calcretes (fossil soils) within the muddy sediments, and the shifting and valley-building activities of mixed tidal and river channels, could have been under the control of relative fluctuations of sea level perhaps on as short a time scale as of the order of 10 4 years. The channellized currents seem typically to have been vigorous and of low to moderate sinuosity.

Clay mineral stratigraphy in the Permo-Triassic red bed sequences of BNOC 72/10-1A, Western Approaches, and the south Devon Coast

AbstractThe Permo-Triassic red bed sequence penetrated in BNOC Well 72/10-1A, located in the Western Approaches, is divided into eight clay mineral zones. The zones are defined by variations in the clay assemblage in which iron- and magnesium-rich chlorite, corrensite and mica occur. This zonal sequence is matched mineralogically and stratigraphically with the pattern of clay mineral variation recorded from the Permo-Triassic sediments of the south Devon coast, some 500 km distant.

Spore-pollen assemblages of the Bowen Basin, Queensland (Australia): Their relationship to the Permian/Triassic boundary

Five palynofloral zones have been recognised from fully cored sections of the Middle Permian to Early Triassic succession of the Bowen Basin, Queensland. In ascending order they are: “Upper Stage 5”; Playfordiaspora crenulata Zone, defined herein; Protohaploxypinus microcorpus Zone, emended; Lunatisporites pellucidus Zone, emended; and Protohaploxypinus samoilovichii Zone. The succession, which is non-marine, commences with coal measures of the Blackwater Group, and continues into a predominantly red bed sequence of the overlying Rewan Formation. Assemblages of the P. crenulata Zone, from the youngest seams of the coal measures, represent a transitional palynoflora, in that they are dominated by the same diverse suites of disaccate pollen grains which are typical of “Upper Stage 5”, and also contain taxa which are more characteristic of the succeeding zones. The recognition of this zone emphasises the continuity of both the stratigraphic and palynofloral succession. Based on palynological evidence, two chronologic anchor points are established within the succession. Assemblages of the P. crenulata Zone are of Middle Permian (latest Chhidruan—?early Djulfian) age, while those of the P. samoilovichii Zone are of Early Triassic (late Dienerian to earliest Smithian) age. Quantitative similarities between assemblages of the three youngest zones allow them to be grouped as a Rewan palynoflora. The major palynofloral change therefore commenced during the Middle Permian. For the remainder of that period, the Rewan flora, as reflected by plant microfossils, became established and continued to flourish in the Triassic. Locally, there is therefore no correspondence between the Permian/Triassic boundary and the coal measures/Rewan Formation boundary, as suggested by earlier authors. The timing of this major palynofloral change, which heralded the Mesophytic flora, is unrelated to marine biota crises which occurred elsewhere at the close of the Permian.

Depositional Environments of Norphlet Formation (Jurassic) of Southwestern Alabama: ABSTRACT

The Norphlet Formation in southwestern Alabama has become a primary target for oil and gas exploration. Isopach data show that Norphlet deposition was affected by a subsiding Mississippi Interior Salt basin, early movement of the Louann Salt, and stable Appalachian ridges and paleohighs, such as the Conecuh ridge and the Wiggins arch. The formation is over 1,000 ft (350 m) thick in parts of Washington, Clarke, Baldwin, and Mobile Counties. The Norphlet thins or is absent over areas of penecontemporaneous salt movement and is absent over the stable Wiggins arch. The Denkman Member of the Norphlet is underlain by a red-bed sequence that grades updip into a conglomerate. The Denkman consists of clean, fine-grained, well-sorted sandstone, including a lower cross-stratified unit and an upper massive unit. The red-bed sequence is mostly sandstone with shale, shaly sandstone, and siltstone at the base. The shales, present in Escambia and Choctaw Counties, were probably deposited as distal portions of alluvial fans. Wadi gravels were deposited adjacent to the Appalachian highlands and grade downdip into the red-bed sequence. Both the gravels and red beds were deposited by sediment-choked braided streams and subjected to reworking and deflation, producing the Denkman Member. The Denkman consists of eolian sands, reworked at the top by the Smackover t ansgression into a massive sand up to 70 ft (21 m) thick. Thin, massive and horizontally laminated units in the cross-stratified sand indicate that narrow interdunes separated the broad dune sands of the Norphlet. A generalized sequence of diagenetic events affecting porosity may have been compaction, quartz overgrowth, and carbonate cementation, and possible selective dissolution of cements followed by deep cementation. End_of_Article - Last_Page 1435------------

Paleomagnetic results from northeastern Mexico: Evidence for large Mesozoic rotations

Preliminary paleomagnetic evidence indicates that a significant part of Mexico rotated 130° counterclockwise during early Mesozoic time, coeval with the opening of the Gulf of Mexico, and that a major structural discontinuity must exist between Mexico and cratonic North America. Measurements made on samples from fourteen isolated outcrops of the Huizachal Group, a red-bed sequence exposed in northeastern Mexico indicate that these sediments were deposited in rifted basins and could range in age from post–Middle Permian to pre–Early Cretaceous. Of samples analyzed from the overlying Zuloaga Limestone (Oxfordian) and the Cuesta del Cura Formation (Aptian-Albian), the oldest yield a paleomagnetic pole position that implies that the sampling area was originally oriented about 130° clockwise relative to stable North America. Progressive counterclockwise rotation continued to Middle Jurassic time. Because of the limited geographical distribution of sampling sites, it is not possible to assess whether most of Mexico shared this rotation or whether the proposed left-lateral motion along major west-northwest–trending faults caused the rotation of a smaller crustal block. In either case, our results strongly suggest a major structural discontinuity in northern Mexico.

Uranium Mineralization in North-Central and Southwestern Oklahoma: ABSTRACT

At least three types of uranium occurrences are known in the study areas. In north-central Oklahoma (Enid 2 × 1° Quadrangle) several uranium-copper shows occur in the Oscar group and Wellington Formation. The mineralization is associated with plant debris and is confined to gray, fine-grained sandstone lenses within a red-bed sequence. The most reasonable source for the Cu and U are the red beds, with Cu and U released by subsurface breakdown of minerals. The maximum Cu and U concentrations are 2.95% and 125 ppm, respectively. In contrast, the Kupferschiefer-type copper deposits in southwestern Oklahoma (Creta and Mangum) contain only up to 12 ppm U. In southwestern Oklahoma (Clinton 2 × 1° Quadrangle) carnotite and tyuyamunite occur in siltstones of the basal part of the Doxey formation. Although the origin of the mineralization is not entirely clear, the common association of uranium with the red-to-gray interfaces may reflect the location of primary ore at a redox interace, since uranium shows little tendency to migrate during oxidation of deposits containing appreciable vanadium. In addition, interpretation of approximately 700 ground-water analyses using the WATEQFC program yielded suggestions as to possible targets for further investigation in the Clinton Quadrangle. The third type of uranium occurrence is associated with End_Page 1495------------------------------ petroliferous areas. Of these, the Cement oil field has produced the only commercial uranium deposit in Oklahoma. Less well known is a small occurrence in Kiowa County (in the Hennessey group) where uranium is associated with pyrobitumen nodules. The nodules contain between 2,225 and 10,110 ppm U. Available field, microscopic, and geochemical evidence suggest that the pyrobitumens are secondary, i.e., alteration products of crude oil. Uranium was provided by ground water. Petroliferous areas in Oklahoma hold the greatest potential for discovery of significant uranium mineralization. Oil-field brines can, in some places, be a useful tool in exploration for such mineralization. While high radium-226 concentrations in natural waters are not a specific indicator of subsurface uranium mineralization, relatively high ratios of Ra-226/Ra-228, Ra-226/Ba, Ra-223/Ra-228, and Ra-223/Ba in such waters from sedimentary terranes may better reflect the presence of uranium accumulations in the subsurface. Available analyses of several radioactive oil-field brines and springs from various parts of Oklahoma indicate that these waters do not meet the above criteria and are not, therefore, surface expressions of buried uranium mineralization. End_of_Article - Last_Page 1496------------

Mineralogy and paragenesis of red-bed copper mineralization in the Lower Permian of South central Kansas

Subeconomic concentrations of copper sulfides within greenish gray shales, argillaceous dolomites, and limestones of the Lower Permian red-bed sequence of south central Kansas have been investigated. The host rocks are similar to those hosting red-bed mineralization in Oklahoma and Texas. Copper sulfides occur principally as replacements of earlier formed diagenetic pyrite. Two distinct, spatially unrelated sulfide assemblages have been identified in drill cores. In the northern portion of the area pyrite is replaced by chalcopyrite and then bornite at shallower depths. In the southern section pyrite is replaced by nearer the surface. The depth zonation strongly suggests that descending fluids produced the Cu mineralization. Electron microprobe studies of the sulfide minerals indicate that bornite, as in other red-bed deposits, is sulfur rich. In addition, true chalcocite (Cu 2 S) was not found in the studied area. Instead, two phases having compositions of Cu (sub 1.78 + or - 0.04) S (similar to anilite) and Cu (sub 1.91 + or - 0.03) S (similar to djurleite) were identified. Thermochemical calculations suggest that continuous depletion of a Cu-rich ore fluid and variations in pH and f (sub O 2 ) are responsible for producing the pyrite-chalcopyrite-bornite and pyrite-chalcocite assemblages.

Stratigraphy and depositional setting of the Sibley Group, Thunder Bay district, Ontario, Canada

The Sibley Group is a Neohelikian (1339 ± 33 Ma, RbSr, 87Rb = 1.42 × 10−11 year−1) red bed sequence located in an elongate basin extending northward for 150 km from Nipigon, Ontario. The lowest unit, the Pass Lake Formation (new name), is 50 m thick and consists of quartz arenite, deposited primarily in a shallow, quiet lacustrine environment. It is overlain by the Rossport Formation (new name), a unit of 135 m thickness consisting of a lower arenaceous red dolomite member, central chert–carbonate and stromatolite member, and an upper argillaceous red dolomite member. The Rossport Formation was deposited in a shallow, highly saline environment, in a basin of fluctuating size. The Kama Hill Formation (new name) is 50 m thick, and consists of purple shale composed of smectite, authigenic microcline, and quartz. It was deposited in a periodically dry mud flat, and is characterized by desiccation cracks, evaporite casts, and mud-chip microbreccias.Both the Rossport and Kama Hill Formations have an increased arenite content near the basin margins. Breccias cut the Rossport Formation and consist of stoped blocks of Rossport and Kama Hill rocks; these breccias are cut by sandstone dykes. The Sibley Group is situated in, and was possibly deposited in, a "failed arm" which radiates from a paleo-plume in the Keweenawan rift valley.

Early palaeozoic palaeomagnetism in Australia I. Cambrian results from the Flinders Ranges, South Australia II. Late Early Cambrian results from Kangaroo Island, South Australia III. Middle to early-Late Cambrian results from the Amadeus Basin, Northern Territory

I. Cambrian results from the Flinders Ranges, South Australia A total of 460 samples from six sequences spanning the Cambrian succession of the Flinders Ranges (Adelaide “Geosyncline”, South Australia) has been analyzed through thermal demagnetization studies. All samples showed a recent field component, generally constituting more than 50% of the initial intensity, which in most cases was removed by 200–400°C. Two characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Cambro-Ordovician age (S-pole at 75.3°E 26.0°N, α 95 = 7.4°, N = 5 localities) interpreted as having been induced by thermochemical activity during a period of enhanced heat flux prior to the Late Cambrian-Early Ordovician diastrophistic phases of the Delamarian Orogeny. 2. (B) A primary magnetic component, which indicates rapid polar motion during the Early Cambrian and a much reduced polar motion during the Middle Cambrian. Representative palaeomagnetic pole positions for the primary component are: 1. (1) Basal Hawker Group (earliest Cambrian): S-pole at 2.3°E 26.7°S, d p = 8.1°, d m = 14.3°, N = 10 (sites). 2. (2) Billy Creek Formation— Wirrealpa Limestone— Aroona Creek Limestone (late Early Cambrian to early Middle Cambrian): S-pole at 20.1°E 37.4°S, d p = 7.2°, dm = 14.4°, N = 11(sites). 3. (3) Basal Lake Frame Group (Middle Cambrian): S-pole at 26.1°E 29.3°S, d p = 6.6°, d m = 13.1°, N =10 (sites). 4. (4) Pantapinna Formation (late Middle Cambrian?): S-pole at 29.2°E 36.4°S, d p = 8.4°, dm = 16.7°, N = 4 (sites). Available data suggest that deposition of the Lake Frome Group beds probably did not continue into the Late Cambrian. II. Late Early Cambrian results from Kangaroo Island, South Australia A total of 108 block samples from a late Early Cambrian red-bed sequence on Kangaroo Island (Adelaide “Geosyncline”, South Australia) has been analysed through thermal demagnetization studies. All samples contained a recent field component of considerable intensity. Two characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Late Cambrian—Early Ordovician age (S-pole at: 75.8°E 17.4°N, d p = 4.2°, d m = 1.9°, N = 54 specimens), attributed to thermochemical activity predating the main folding phases of the Delamarian Orogeny. 2. (B) A primary magnetic component corresponding to a S-pole position at 15.1°E 33.8°S (d p = 6.2°, d m = 12.3°, N = 16 sites). Both the primary and the secondary magnetic component are in very good directional agreement with the magnetization pattern from the correlated Billy Creek Formation of the Flinders Ranges (I). Consequently, noticeable rotational movement since late-Early Cambrian times between Kangaroo Island and the northwestern part of the Adelaide “Geosyncline” can be ruled out. III. Middle to early-Late Cambrian results from the Amadeus Basin (Northern Territory) A total of 328 samples from a Middle Cambrian red-bed succession and a Middle to early-Late Cambrian carbonate succession in the Amadeus Basin (Central Australia) have been analyzed through thermal demagnetization studies. All samples contained a recent field component of considerable intensity which became broken down, respectively below 200°C in the carbonate samples and between 300°C and 500°C in the red-bed samples. Another recent field component, broken down between 600°C and 675°C, was noted in some of the red-bed samples. Three characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Late Devonian—Early Carboniferous age (S-pole at 110.5°E 46.9°S, N = 2 localities) which predates the main folding phase of the Early Carboniferous Alice Springs Orogeny. 2. (B) Another secondary magnetic component (S-pole at 60.8°E 33.8°N, N = 2 localities) which is very similar to a thermo-chemically induced Cambro-Ordovician magnetic component, noted in rocks from the Adelaide “Geosyncline”. 3. (C) A primary magnetic component which suggests reduced polar motion during the Middle Cambrian (S-pole at 16.6°E 35.0°S, α 95 = 7.4°, N = 5 sites) and rapid polar motion during the early-Late Cambrian (S-pole at 34.9°E 14.9°N, d p = 6.3°, d m = 11.6°, N = 9 specimens).

Hydrocarbon Potential of Matilija Sandstone, an Eocene Sand-Rich, Deep-Sea Fan and Shallow-Marine Complex, California: ABSTRACT

The Matilija Sandstone Member, exposed in the Santa Ynez Mountains, California, records a major regressive event in the Eocene Santa Ynez basin in which turbidites were deposited in the basin and subsequently covered by shallow-marine complexes. Despite thick favorable source beds and generally good initial reservoir characteristics, the Matilija sandstone is not a productive unit in the basin. Lowered reservoir rock permeability (< 1 md) and porosity (0-10%) are due to early compaction, cementation, and diagenesis. The lower part of the Matilija sandstone is a 700-m-thick sand-rich deep-sea fan complex which overlies basin plain and turbidite deposits (Juncal shale). The Matilija sandstone consists of anastomosing outer-fan depositional lobes overlain by channelized middle- and inner-fan deposits. Cross-bedded sandstone, red-bed, and carbonate-evaporite sequences overlie the turbidites. Matilija sandstone deposition closed with rapid transgression which culminated in the deposition of basin plain and turbidite deposits (Cozy Dell shale). The Matilija sandstone lower deep-sea fan complex has a high sandstone and shale ratio (4:1) and consists of submature arkoses of facies B. The average sandstone is medium grained, moderately sorted, subangular, massive, and contains 40% quartz, 35% feldspar (about equal amounts of potassium and plagioclase feldspars), 10% lithic fragments (mostly granitic and volcanic types), and smaller amounts of mica, chert, and heavy minerals. Early compaction, carbonate cementation, and authigenic pore-lining chlorite, albite, and quartz have reduced the initial porosity and permeability. Minor secondary fractures are the only effective porosity in these rocks now. End_of_Article - Last_Page 740------------

Pennsylvanian and Early Permian Paleogeography, Southern Colorado Plateau and Vicinity: ABSTRACT

Abstract Pennsylvanian and Lower Permian sedimentary rocks of the southern Colorado Plateau have been the subject of much controversy concerning their correlation and origin. Correlation of rock sequences and major unconformities from the fossiliferous basins onto the adjacent shelves, where relatively unfossiliferous rocks were deposited, has provided a new interpretation in terms of regional stratigraphy and depositional analysis. The Supai Group was deposited in the Grand Canyon Embayment adjacent to the Cordilleran Geosyncline. Sediments of Morrowan, Atokan, Virgilian and Wolfcampian ages were deposited in a variety of offshore marine and shoreline environments. A persistent southwest-to-northeast-trending unstable area, herein named the Sedona Arch, confined all Supai sediments, except Virgilian, to the Grand Canyon and western Mogollon Rim regions; the other strata are truncated beneath the sub-Virgilian unconformity or undergo significant facies change across the arch. The Naco Group of Desmoinesian, Missourian, and Virgilian age was deposited on the Mogollon Shelf adjacent to the Pedregosa Basin. Except for Virgilian strata, these dominantly marine rocks are also truncated beneath the sub-Virgilian unconformity across the Sedona Arch. Thus the bulk of Supai and Naco rocks are not as closely related as most previous workers had thought. Mostly marine rocks of the Molas Formation and Hermosa Group were deposited in the Paradox Basin. The northwest-to-southeast-trending Kaibab Arch provided the barrier between these rocks and the Supai and Naco to the south. Except for several transitional units, the Supai, Naco, and Hermosa Groups were deposited in distinctly separate basins and the units do not generally intertongue. The Wolfcampian Esplanade and Cedar Mesa Sandstones and Halgaito Formation form a distinct eastward-thinning wedge of rocks west of the Sedona Arch. These high-energy marine shoreline and supratidal units grade westward into thick marine carbonate sequences of the Cordilleran Geosyncline. A widespread redbed sequence comprising the Hermit and Organ Rock Formations was deposited across the entire study area. The continental and possibly tidal deposits represent a sharp change in depositional regime from the underlying and overlying sandstone deposits. The youngest sequence of deposition studied consists of sandstone, redbed, evaporite, and carbonate rocks of the Schnebly Hill Formation and DeChelly Sandstone. These units are centered around the Holbrook Basin and represent a complex of shallow marine, high- and low-energy shoreline, eolian, and sabkha depositional environments. This depositional sequence was confined to an area east of Grand Canyon and south of the San Juan River by the Sedona and Kaibab Arches. Both formations grade upward or intertongue with the overlying Coconino Sandstone and related rocks.

Palaeogeographical evolution of the Lower Old Red Sandstone basin in the western Midland Valley

Synopsis Lower Old Red Sandstone rocks in the western Midland Valley form a red-bed sequence of probable alluvial origin. They accumulated in a NE-SW elongated basin linked in its early stages of development with similar depositional conditions in North Argyll. The basin, whilst changing its morphology with time, maintained a regional south-westerly dipping palaeoslope with deposition in a longitudinally draining floodplain environment replaced by laterally draining alluvial fans close to the inferred northern margins of the basin. A lithostratigraphy is proposed within which rocks are attributed to four distinct groups corresponding to those recognized in the NE Midland Valley. Within this lithostrarigraphical framework changes in sedimentation, palaeogeography and tectonic controls can be studied through time. The sedimentation time-trends suggest that along the northern margin deposition was controlled by faulting which, west of Loch Lomond, probably occurred along small normal faults shifting their location with time. Any extension of the Highland Boundary Fault into this western area was ineffective as a major feature either controlling deposition or defining the basin margins.

Detrital magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountain, Arizona

Specularite, apparently formed and in part altered to red hematite before deposition, carries much of the stable magnetization in sandstone, siltstone, and claystone of the Moenkopi Formation (Lower Triassic) at Gray Mountain, northern Arizona. High‐temperature exsolution textures seen in polished section indicate that some, if not much, of the specularite is detrital. Progressive thermal and chemical demagnetization analyses indicate that the most stable magnetization resides in specularite. Last, a systematic depositional inclination ‘error’ is seen in both normally and reversely polarized sandstone. Part of its cause was the deposition along fine bedding laminations of subtabular grains of specularite carrying a strong direction of magnetization in the plane of tabularity. Magnetic inclinations are significantly shallower in sandstone than in siltstone, and the amount and sense of inclination error are controlled in detail by the dip of the horizontal and cross laminations. The foregoing leads to the conclusion that the stable magnetization is a detrital magnetization acquired during the course of deposition and that the Moenkopi Formation thus is suitable for magnetostratigraphy. Neither the formation of specularite nor the acquisition of the stable magnetization appears to be the consequence of prolonged and complicated postdepositional chemical alteration, a process invoked by some workers for formation and magnetization of ancient red beds. Because specularite is common in other ancient red bed sequences, their stable magnetizations also may be partly or largely detrital.

Petrology and Diagenesis of Arc-Derived Lithic Sandstones--Wagwater Trough, East-Central Jamaica: ABSTRACT

The Wagwater trough is a fault-bounded block of lower Tertiary sedimentary and volcanic rocks that cuts across east-central Jamaica in a southeast direction. Detrital sandstone and conglomeratic sequences within this block include the early Eocene and Paleocene(?) Wagwater and Richmond Formations. The Wagwater Formation is a red-bed sequence of breccias, conglomerates, sandstones, and mudstones with minor amounts of limestone and gypsum deposited as a subaerial fan-delta complex. Sedimentary rocks of the Wagwater Formation are laterally equivalent to sandstones, conglomerates, shales, and limestones of the Richmond Formation, which were deposited in submarine-slope and fan environments. Lithic arenites are the dominant sandstone type within the trough. Principal framework constituents include volcanic and carbonate lithic fragments, plagioclase, quartz, and fossil fragments with minor amounts of plutonic lithic fragments and opaque heavy minerals. Authigenic minerals are abundant in some sandstones and include phyllosilicates (chlorite and clay minerals), calcite, and iron oxide. The progressive sequence of diagenetic features from earliest to latest in marine sandstones of the Richmond Formation is: (1) development of clay coats around framework grains, (2) precipitation of calcite pore-fill cement, (3) development of a second clay coat on calcite cement in incompletely filled pores, (4) crystallization of radiating pore-fill chlorite after development of clay coats in either stage 1 or stage 3, and (5) late-stage calcite replacement. Despite minor differences in diagenetic features, the Richmond sandstones are similar to other arc-derived sandstones from five other basins. These separate occurrences suggest that sands deposited in arc basins have the same compositional range, and that postdeposition processes produce a common diagenetic sequence. End_of_Article - Last_Page 447------------

Keweenawan turbidites in Michigan (deep borehole red beds): A foundered basin sequence developed during evolution of a Protoceanic Rift System

A 1538‐m pre‐Mt. Simon (Upper Cambrian) red bed sequence, encountered in the Gratiot County deep borehole, consists of undeformed recurring beds of submature and immature arkosic sandstone, 5–85 cm thick, that grade upward into mudstone units, 3 cm to 3.8 m thick. Similar facies are recorded by a red bed sequence penetrated in a well on Beaver Island, 260 km to the north, and both sequences occur within the mid‐Michigan gravity high. We suggest penecontemporaneous deposition by sediment gravity flows, low‐velocity clear water currents, and pelagic fallout on submarine fans forming in an elongate, broadly subsiding protoceanic basin which foundered isostatically following arrestment of Keweenawan rifting and production of oceanic lithosphere. We further suggest that protoceanic rifting in the Lake Superior region and that along the mid‐Michigan gravity anomaly were approximately contemporaneous and that both areas underwent a similar sequence of thermotectonic volcanism, deformation, and sedimentation, followed by passive foundering and shallow marine and basinal turbidite sedimentation.

Paleosol caliche in the New Haven Arkose, Newark Group, Connecticut

The New Haven Arkose is a 1500–2500 m sequence of fluvial red beds of Late Triassic and probably Early Jurassic age that accumulated in a tropical rift valley. Streams flowing from highlands along the valley deposited braided channel sand interbedded with overbank mud. At most outcrops in Connecticut, some of the beds of red mudstone and sandstone contain paleosol caliche with densely packed root casts and rhizoconcretions. These calcified mudstones and sandstones commonly show the initial stages of calcification where subspherical to vertically elongate calcite nodules with crystic plasmic (microspar) fabric replace up to 50% of the host rock. In more mature paleosols, coalescing calcite nodules replace 50–95% of the rock, accompanied by paleosol mosaics of pedotubules, peds, and laminated crystallaria. The best developed caliche profiles are capped by thin (0–10 cm) layers of nearly pure limestone with complex fabrics of nodules, microspar, veins, caliche clasts, laminae, and calcite cement. The caliche profiles accumulated when rates of sedimentation were low. Much of the time the tropical paleoclimate was semiarid, perhaps with 100–500 mm of seasonal rain and a long dry season.