Augustus K. Armstrong

Carboniferous Biostratigraphy, Prudhoe Bay State 1 to Northeastern Brooks Range, Arctic Alaska

The 3,000-ft-thick Carboniferous section in Atlantic Richfield/Humble Prudhoe Bay State 1 is divided into two groups. The older rocks in the section, Endicott Group, are 1,200 ft thick and unconformably overlie argillites of the pre-Mississippian Neruokpuk Formation. Within the Endicott Group, the Kekiktuk Conglomerate is 600 ft of gray to dark-gray sandstone, shale, and thin-bedded coal, deposited in a paralic environment. The overlying Kayak(?) Shale is 620 ft of quartzitic and calcareous red sandstone, siltstone, shale, and arenaceous to ooid limestone; it represents transition to marine sedimentation and contains microfossils of Zone 16s (Chesterian). Overlying the Endicott Group, the Lisburne Group is 1,770 ft thick. Within the Lisburne Group, the Alapah L mestone, 1,050 ft thick, includes arenaceous bryozoan-echinoderm wackestone and packstone to lime mudstone and microdolomite with gypsum. The age of the Alapah Limestone ranges from Zones 16s through 18 (Chesterian). The overlying Wahoo Limestone, 720 ft thick, is a series of cyclic glauconitic-bryozoan-echinoderm-ooid packstone and grainstone capped by lime mudstone and arenaceous microdolomite. These are interpreted to represent cyclic deposition of tidal-flat lime mud developed directly over ooid tidal bars. The age of the Wahoo Limestone ranges from Zone 20 to Zone 21 (Pennsylvanian). In comparison with the section penetrated by this well the outcrop data 75-112 mi southeast show that the Endicott Group is diachronous because, in the Franklin Mountains, it has a lithology similar to that found in the Prudhoe well but contains (Meramecian) Zone 11 marine microfossils. During the Meramecian the Sadlerochit Mountains were high; the Endicott Group is either absent or less than 50 ft thick here. The Alapah Limestone of Prudhoe Bay State 1 is more dolomitic and arenaceous than its equivalent in the Franklin or Sadlerochit Mountains. The Wahoo Limestone of the Prudhoe Bay subsurface is similar in lithology and in environments of deposition to the outcrop sections in the Sadlerochit Mountains.

Carboniferous Carbonate Depositional Models, Preliminary Lithofacies and Paleotectonic Maps, Arctic Alaska

Two distinct environmental suites can be delineated for the shallow-marine Carboniferous rocks of Arctic Alaska. One is a marine transgressive suite, typified at its base by a paralic clastic sequence followed by nearshore shales and siltstones, and overlain by spiculitic carbonate rocks. This transgressive clastic-marine carbonate sequence is associated with regional Mississippian transgression. Developed above this is a carbonate platform sequence characterized by very low terrigenous clastic content and shallow-water cyclic carbonate sedimentation. The outer margins of the shelf did not support reef-forming organisms. Carboniferous sedimentation in Arctic Alaska represents a northward marine transgression over an irregular surface with residual stable positive areas. In the northeast, the Sadlerochit high was not submerged until early Chester time. In the west-central foothills region, the Nuka high is a long, linear, east-trending positive area in the subsurface; it may have been the source for the Mississippian feldspathic clastic rocks of the Nuka Formation. The Nuka high may have remained a mildly active positive element during the Mississippian. A northeast-trending negative area, the Canning sag, stretches from the Prudhoe Bay region southwest to the Sagavanirktok River region of the Brooks Range, where there is a 3,000-ft-thick accumulation of shallow-water Carboniferous carbonate rocks. The Lisburne Group, at many outcrops in the Brooks Range, was deposited as a complex of cyclic shallow-water carbonate sediments in an arid or semiarid climate. A typical cycle grades from marine ooid-crinoid grainstone to supratidal microdolomite. Composite cyclic carbonate sequences may be thousands of feet thick in outcrop. Outcrop studies indicate that thick sequences that have been dolomitized by reflux brines may have reservoir qualities in the subsurface of the north slope.

Foraminiferal Zonation and Carbonate Facies of Carboniferous (Mississippian and Pennsylvanian) Lisburne Group, Central and Eastern Brooks Range, Arctic Alaska

The Lisburne Group carbonate rocks of the central and eastern Brooks Range contain foraminiferal assemblages assigned to zones of late Tournaisian (Osage) to early Moscovian (Atoka) age. Representatives of both Eurasiatic and American cratonic microfaunas permit correlation with the original Carboniferous type sections in western Europe as well as with the standard Mississippian and Pennsylvanian sequences in the Mid-Continent region of North America. Correlation anomalies in the lower part of the sequence are discussed.

Endemism and similarity indices: Clues to the zoogeography of North American Mississippian corals

The distribution of coralliferous facies and the degrees of endemism and generic similarities of Mississippian coral faunas permit recognition of five zoogeographic provinces and five zoogeographic sub-provinces in North America: Alaskan province, Pacific Coast province (including Northern and Southern sub-provinces), Western Interior province (including Northern, Central, and Southern sub-provinces), Southeastern province, and Maritime province. Analysis of indices of endemism and similarity suggest the following major conclusions: (1) Zoogeographic regions on the periphery of the North American continent (Alaskan, Pacific Coast, and Maritime provinces) had favorable connections for migration to other coralliferous areas of the world, which permitted maximum gene flow; (2) zoogeographic regions in the interior of the North American Continent (Western Interior and Southeastern provinces) were relatively isolated genetically and were characterized by coral faunas having low to high endemism throughout Mississippian time; (3) gene flow was highest along continuous shallow-water carbonate shelves and was impeded by areas of terrigenous sedimentation and areas of deeper water; and (4) similarities between faunas of different zoogeographic regions generally tend to vary inversely with the migration-route distance between these regions, but other factors that affected gene flow modified the distribution patterns significantly.

Comparison of hydrothermal alteration of carboniferous carbonate and siliclastic rocks in the Valles caldera with outcrops from the Socorro caldera, New Mexico

Abstract Continental Scientific Drilling Program (CSDP) drill hole VC-2B [total depth 1761.7 m (5780 ft); maximum temperature 295 °C] was continuously cored through the Sulphur Springs hydrothermal system in the western ring-fracture zone of the 1.14 Ma Valles caldera. Among other units, the hole penetrated 760.2 m (2494.1 ft) of Paleozoic carbonate and siliciclastic strata underlying caldera fill and precaldera volcanic and epiclastic rocks. Comparison of the VC-2B Paleozoic rocks with corresponding lithologies within and around the 32.1 Ma Socorro caldera, 192 km ( 119 miles) to the south-southwest, provides insight into the variability of alteration responses to similar caldera-related hydrothermal regimes. The Pennsylvanian Madera Limestone and Sandia Formation from VC-2B preserve many of the sedimentological and diagenetic features observed in these units on a regional basis and where unaffected by high temperatures or hydrothermal activity. Micrites in these formations in VC-2B are generally altered and mineralized only where fractured or brecciated, that is, where hydrothermal solutions could invade carbonate rocks which were otherwise essentially impermeable. Alteration intensity (and correspondingly inferred paleopermeability) is only slightly higher in carbonate packstones and grainstones, low to intermediate in siltstones and claystones, and high in poorly cemented sandstones. Hydrothermal fracture-filling phases in these rocks comprise sericite (and phengite), chlorite, allanite, apatite, an unidentified zeolite and sphene in various combinations, locally with sphalerite, galena, pyrite and chalcopyrite. Terrigenous feldspars and clays are commonly altered to chlorite and seriate, and euhedral anhydrite “porphyroblasts” with minor chlorite occur in Sandia Formation siltstone. Fossils are typically unaltered, but the walls of some colonial bryozoans in the Madera Limestone are altered to the assemblage chlorite-sericite-epidote-allanite. La, Ce and Nd are present in an unidentified hydrothermal mineral occurring throughout much of the VC-2B Pennsylvanian sequence. Carboniferous carbonate and siliciclastic formations within and around the Socorro caldera show a similar style of alteration and mineralization to their Valles caldera counterparts, but by contrast locally host commercial, caldera-related, base-metal sulfide deposits. As in the Valles rocks, mineralization and alteration in those of the Socorro caldera were strongly controlled by porosity. Unless disrupted by fractures, breccias, or karst cavities ( not identified in Valles caldera drill holes), the rocks remained relatively unaltered. Where these features allowed ingress of mineralizing hydrothermal solutions, base-metal sulfides and rare-earth-element-bearing minerals were precipitated.

The Mississippian System of New Mexico and Southern Arizona: ABSTRACT

Lower Mississippian rocks of New Mexico and southern Arizona (pre-zone 7 Tournaisian age) are unconformable on rocks of Late Devonian to Precambrian age. Mississippian rocks were deposited during transgression on a surface of low relief. Tournaisian transgression began in southern Arizona, depositing the Escabrosa Limestone and, in southwestern New Mexico, the Keating (207 m), Caballero (18 m), and Lake Valley (180 m) Formations. At the end of Tournaisian time, epicontinental seas flooded southern and central Arizona, depositing the younger parts of the Escabrosa and Redwall Limestones. Osagean seaways extended to central and northern New Mexico depositing Kelly (35 m) and Espiritu Santo (35 m) Formations. The Espiritu Santo consists of subtidal to supratidal quartz sands one and carbonate rocks. Zuni Highlands and Pedernal Highlands formed two low islands. The end of the Tournaisian is marked by marine regression, regional uplift, and erosion. Major regional marine transgression in early Visean is represented by parts of Escabrosa Limestone of southern Arizona, massive encrinites of the Hachita Formation (107 m) in southwestern New Mexico, basin carbonate rocks of the lower part of the Rancheria Formation (46 m) in south-central New Mexico, and the subtidal Tererro Formation (18 m) in north-central New Mexico. The Cowles Member (10 m) of the Tererro Formation indicates that sedimentation ceased in northern and central New Mexico in late Visean time. In southwestern New Mexico, the Paradise Formation (134 m) represents shallow-marine sediments and ranges rom zone 15 into zone 19 (late Visean and Namurian). The Rancheria Formation (69 m) and the Helms Formation (50 m) of south-central New Mexico are deep-water facies of the Paradise Formation. Pennsylvanian sedimentary rocks in southern Arizona and in New Mexico truncate Mississippian sedimentary rocks of Namurian, Visean, and Tournaisian age. End_of_Article - Last_Page 671------------

Carboniferous Colonial Rugose Corals, Biostratigraphy, and Paleoecology, Lisburne Group, Arctic Alaska: ABSTRACT

The Lisburne Group of arctic Alaska contains coral faunas of Osagian (Early Mississippian) to Atokan (Middle Pennsylvanian) age. Beds of Osagian age have a small fauna of solitary and tabulate corals. Beds of Meramecian and very earliest Chesterian ages contain a large fauna of Ekvasophyllum spp., Faberophyllum spp., Diphyphyllum klawockensis Armstrong, D. nasorakensis Armstrong, Lithostrotion (Siphonodendron) dutroi Armstrong, L. (S.) sinuosum (Kelly), L. (S.) warreni Nelson, L. (S.) lisburnensis Armstrong, Lithostrotion reiseri Armstrong, Lithostrotionella niakensis Armstrong, L. banffensis (Warren), L. mclareni (Sutherland), L. birdi Armstrong, L. pennsylvanica (Shimer), Thysanophyllum astraeiforme (Warren), T. orientale Thomson, Sciophyllum lambarti Harker and McLaren and S. alaskaensis Armstrong. Corals are rare in beds of younger Chesterian age; they include Lithostrotionella aff. L. mclareni (Sutherland), Lithostrotion (S.) ignekensis Armstrong, Syringopora spp., and a few solitary corals. Pennsylvanian (Atokan) beds of the Lisburne Group contain Lithostrotionella wahooensis Armstrong, Corwenia jagoensis Armstrong, a thick-welled syringoporoid, and Michelinia sp. The Lisburne Group limestones are cyclic and were deposited on a slowly subsiding carbonate platform. Colonial corals of Meramecian and Atokan ages are present in carbonate rocks associated with shallow-water shoaling facies. The scarcity of corals in carbonate rocks of Osagian, Chesterian, and Morrowan ages is attributed to regional temperature or salinity changes that inhibited their growth. Beds of Atokan age contain more calcareous algae and Foraminifera, indicating warmer waters. Paleoecologic analysis of the carbonate beds associated with the colonial corals of Atokan age indicates that the corals lived in clear, agitated water between oolitic tidal flats. Carboniferous corals are not known to have formed reef-like masses in arctic Alaska. End_of_Article - Last_Page 768------------

Foraminifera and Rugose Coral Zones of Mississippian-Pennsylvanian Lisburne Group, Brooks Range, Arctic Alaska: ABSTRACT

Thin-section studies of outcrop samples from the Brooks Range show that the Lisburne Group ranges in age from Osagean (Early Mississippian) to Atokan (Middle Pennsylvanian), and can be divided into 15 microfossil zones (Bernard Mamets zones 8 through 21). Although the foraminiferal fauna is impoverished, the resolution of the foraminiferal zones is finer than zones based on rugose corals. Also, the Foraminifera are in a much wider range of carbonate paleoenvironments than are the rugose corals. Colonial rugose corals are relatively abundant in the Lisburne Group in shelf carbonates of Meramecian and Atokan ages. These beds can be subdivided by use of corals into faunal zones and used for regional correlation within the Cordillera of North America. In the Brooks Range, Lisburne Group shelf carbonates of Osagean, Chesteran, and Morrowan ages are relatively poor in colonial rugose corals and are zoned exclusively by Foraminifera. End_of_Article - Last_Page 559------------