Donald P. Elston

Paleomagnetic poles and polarity zonation from Cambrian and Devonian strata of Arizona

Abstract Basal Paleozoic Tapeats Sandstone (Early and Middle Cambrian) in northern and central Arizona exhibits mixed polarity and a low-latitude paleomagnetic pole. Carbonates of Middle and early Late Cambrian age, and directly superposed carbonate and carbonate-cemented strata of latest Middle(?) and early Late Devonian age, are characterized by reversed polarity and high-latitude poles. The high-latitude Middle Cambrian pole, which appears to record a large but brief excursion of the polar wandering path, is considered provisional pending additional work. The Devonian data from Arizona indicate that a shift of the pole to a “late Paleozoic” position had occurred by Middle Devonian time.

Age and correlation of the late Proterozoic Grand Canyon disturbance, northern Arizona

A structural disturbance that ended deposition of Chuaria -bearing marine shale of middle Proterozoic age in northern Arizona is recorded by strata within the Sixtymile Formation at the top of the Grand Canyon Supergroup. The disturbance involved marine emergence and uplift, accompanied by block faulting with as much as 3.2 km of structural displacement. It was the most severe episode of structural deformation to affect either the Proterozoic or Phanerozoic strata of the Grand Canyon, but it was markedly less severe than an earlier deformation which resulted in meta-morphism and intrusion of the underlying 1,700-m.y.-old crystalline basement. Two groups of disturbed K-Ar ages obtained from the Proterozoic rocks, 930 ± 25 m.y. and 823 ± 26 m.y., appear to broadly reflect the time of uplift and faulting of the Grand Canyon disturbance. The older age was obtained from mineral analyses of ∼ 1,150-m.y.-old sills and 1,700-m.y.-old crystalline basement buried at a depth of about 4 km at the time, of disturbance. The younger age was obtained from whole-rock analyses of ∼1,100-m.y.-old lava flows buried only half as deeply at the time of the disturbance. A high consistency of ages (values within about 10%) was obtained from both mineral and whole-rock samples having widely varying K 2 O contents, suggesting that little differential loss of Ar has occurred since a time of general resetting. Moreover, scatter on an 40 Ar/ 39 Ar isochron and a disturbed pattern of incremental heating ages indicate that the Ar clock in minerals from the diabase sills was not completely reset. This implies that the Ar clock in the lava flows was more nearly if not completely reset, presumably because of a lower Ar retentivity of the cryptocrystalline matrix of the flows. The 823 m.y. age from the lavas thus is believed to generally reflect a time of cooling and Ar retention accompanying the Grand Canyon disturbance. Strata of the upper Grand Canyon Supergroup correlate paleontologically and paleomagnetically with sedimentary rocks and isotopically dated intrusive rocks of the Little Dal Group of the Mackenzie Mountains Supergroup (northwest Canada), and with sedimentary rocks of the Uinta Mountain Group (Utah and Colorado). An apparent correlation also exists with some poles reported from crystalline rocks of the Grenville Province of eastern North America, and this correlation accords with the broad range of disturbed K-Ar ages from the Grand Canyon (950 to 800 m.y.). Intrusions emplaced after deposition of the Chuaria -bearing Little Dal Group, very near the top of the Mackenzie Mountains Supergroup of Canada, are geologically well controlled and have an internal Rb-Sr isochron age of 770 ± 20 m.y. Intrusion in the Mackenzie Mountains preceded or accompanied deposition of basaltic lava flows and sedimentary strata that accumulated during a time of block faulting, and these strata are in turn overlain by glacio-genic deposits assigned to the late Proterozoic Windermere Supergroup. On isotopic-age, paleontologic, and paleomagnetic grounds, the structural disturbance in the Grand Canyon thus appears to correlate, with post–Little Dal to early Windermere faulting in northwest Canada, a faulting that in turn has been correlated with the disturbance called the “East Kootenay orogeny” in the southern cordillera of Canada. The disturbance in the Grand Canyon is assigned a nominal age of 823 m.y. from an average of the reset K-Ar dates for the Cardenas Lavas. This age would seem to imply that the onset of structural activity occurred somewhat earlier in Arizona than in northwest Canada. If such is the case, data from the Grand Canyon and northwest Canada provide an age range of about 820 to 770 m.y. for a structural disturbance in western North America that separates the deposition of middle Proterozoic from late Proterozoic strata. This range brackets the nominal 800 m.y. age assigned to the boundary between Proterozoic Y and Z rocks on a geologic time scale adopted by the U.S. Geological Survey in 1980. Deposition of the Sixtymile Formation in the Grand Canyon, which accompanied and which also appears to have postdated the disturbance, is assigned to the late Proterozoic. The structural disturbance in the western United States, here called the “Grand Canyon–Mackenzie Mountains disturbance,” was marked by a westward shift in the apparent polar wandering path. A somewhat similar westerly shift is seen in some of the reset poles reported from rocks of the Grenville Province, but a strong southerly shift also is seen, which has given rise to an apparent southerly track (and a postulated loop) in the Grenville polar path. The southerly shift is not seen in the stratigraphically controlled polar path from the western cordillera, which temporally overlaps the age range assigned to the Grenville poles. Although the apparent southerly shift unquestionably is present in the Grenville paleomagnetic data, and also is seen in poles from Grenville-age rocks from Fennoscandia, we suggest that the shift may be an artifact recording the uncorrected effects of structure and structural rotation. The Grenville terrane very likely was subjected to at least some structural deformation at the time of a wide-ranging disturbance beginning about 820 m.y. ago. It is a disturbance that is reported to have affected adjacent rocks in the Appalachian region of eastern North America. Along the western margin of the craton, the. disturbance gave rise to block-fault mountains and to the Cordilleran miogeocline. We suggest that this mountain-making event was the terminal event of the Grenville orogeny.

Cretaceous-Eocene (Laramide) landscape development and Oligocene-Pliocene drainage reorganization of transition zone and Colorado Plateau, Arizona

Landscape development of central and northern Arizona can no longer be ascribed mainly to events of Miocene and Pliocene age. New information on the age and distribution of older Cenozoic deposits has led to the recognition of a regional Cretaceous-Paleocene(?) surface of erosion that conforms to major elements of the present topography and to the recognition that a formerly thick deposit of gravel accumulated on this regional surface of erosion. These relations cast new light on the history of evolution of the landscape and indicate a much greater age for the main landscape elements and a more complicated and prolonged history of erosion and deposition than has been previously supposed. The timing of events postulated for development of drainage on the Colorado Plateau can now be compared and partly reconciled with events recognized in the adjacent closely related Mountain Region (Transition Zone) of central Arizona. As a consequence of Late Cretaceous-Paleocene (Laramide) compression, central and northern Arizona underwent at least 1200 m of uplift, documented by paleochannels cut into erosionally truncated Paleozoic strata on the Hualapai Plateau of the southwestern Colorado Plateau. During this time, a highly irregular erosion surface was developed on Proterozoic rocks across the Transition Zone south of the Mogollon Rim, the scarp of the Mogollon Rim was eroded to its present height (600–900 m), and an extensive stripped surface was developed on resistant upper Paleozoic strata north of the rim. Deposition of several hundred meters of Paleocene-Eocene “Rim gravels” derived from highlands south and west of the region followed, covering much of the Cretaceous-Paleocene erosion surface. Nearly complete burial of the rim is suggested by the distribution of remnants of the Rim gravels across the erosional scarps and on high plateau areas north of the rim. A second increment of uplift, apparently occurring in late Eocene time and apparently recorded by a series of fission track cooling ages from the Marble and Grand canyons, is inferred to have been responsible for ending deposition of the Rim gravels, for initiating differential uplift of contemporaneous deposits (Canaan Peak and Claron formations) to their positions in the high plateaus of central Utah, and for causing the drainage reorganization required to explain the extensive removal of Rim gravels from much of the region. A southerly flowing ancestral Verde River related to the drainage reorganization removed much of the older gravel cover from the Transition Zone of central Arizona, resulting in a younger regional erosion surface having 600–900 m of relief, a surface closely approximating the Cretaceous-Paleocene erosion surface. Late Oligocene and early Miocene rocks locally rest unconformably on remnants of Rim gravels in the Transition Zone, indicating that the second episode of regional erosion had been completed by late Oligocene time. North of the Mogollon Rim, a west flowing(?) ancestral Colorado River is inferred to have become established on the Rim gravels, draining the interior parts of the Colorado Plateau and transporting detritus off the plateau. Exhumation of the Mogollon Rim and development of 600–900 m of topographic relief in the Transition Zone by an ancestral Verde River system suggests the potential for a comparable, coeval entrenchment of an ancestral Colorado River in Paleozoic strata north of the Mogollon Rim. Regional extension and volcanic activity ensued in late Oligocene to Pliocene time. The Oligocene erosion surface in the extensional basins of central Arizona became largely concealed by Miocene and Pliocene deposits as the Neogene climate became drier. In late Miocene and Pliocene time, perennial streams appear to have been lacking, transport of detritus appears to have been principally by flash flooding, little or no detritus appears to have been removed from the region, and much of the precipitation presumably moved by groundwater flow through the deposits of aggradation. A coeval episode of aggradation in the Grand Canyon is suggested by deposits that appear to have once choked much of the canyon. If this event parallels the episode of late Miocene and Pliocene aggradation recorded east, south, and west of the Grand Canyon, the Colorado River could have been incised to its present level by late Miocene time. A return to wetter conditions in late Pliocene time presumably was responsible for renewed erosion and reexcavation of older drainages and basins. An understanding of this Tertiary structural, erosional, and depositional history can be important for the geological analysis of geophysical transects across the region.

Declination and inclination errors in experimentally deposited specularite-bearing sand

Abstract Naturally disaggregated specularite-bearing sandstone from the Triassic Moenkopi Formation, artificially deposited in controlled magnetic fields of ∼5 × 10−2 mT, acquires a stable remanent magnetization that has systematic errors in inclination and declination. Inclinations about 12° shallower than the applied fields are produced by deposition on a horizontal surface in still water. Deposition from flowing water on a surface inclined 6–10° results in inclination errors of as much as 20°. Water flowing obliquely to the applied field results in declination errors of about 10°, with declinations systematically rotated toward the upstream direction of current flow. These experimental results indicate that specularite-bearing sediment responds to the earths field in a manner similar to magnetite-bearing sediment, and support observational evidence for a primary magnetization of depositional origin in specularite in red beds of the Moenkopi Formation.

Paleomagnetism of some Precambrian basaltic flows and red beds, Eastern Grand Canyon, Arizona

Lava flows and red sandstone beds near the middle of the Upper Precambrian Grand Canyon Series exhibit stable remanent magnetization. The beds are about 1000 m stratigraphically above rocks of the Grand Canyon Series for which paleomagnetic poles have been reported. All specimens were subjected to stepwise thermal (200°–700°C) or alternating field (25–5000 Oe) demagnetization for the determination of characteristic magnetization. The pole for two flows and an intercalated sandstone bed of the Cardenas Lavas of Ford, Breed and Mitchell (upper Unkar Group), is at 174.6W, 0.4N ( N = 10, K = 50, α 95 = 6.9°). The pole for a weathered zone developed across the Cardenas Lavas is at 167.8W, 49.4N ( N = 5, K = 79, α 95 = 8.6°). The pole for directly overlying sandstone of the Nankoweap Formation of Maxson is at 174.4E, 12.5N ( N = 6, K = 105, α 95 = 6.6°). These poles lie on or near, and appear to follow, part of an apparent polar wandering path recently proposed for the Precambrian of North America by Spall. If the fit is not accidental, little or no rotation has occurred between north-central Arizona and parts of the North American continent used to define the proposed path.

Unconformity at the Cardenas-Nankoweap contact (Precambrian), Grand Canyon Supergroup, northern Arizona

Red-bed strata of the Nankoweap Formation unconformably overlie the ∼l,100-m.y.-old Cardenas Lavas of the Unkar Group in the eastern Grand Canyon. An unconformity and an apparent disconformity are present. At most places the upper member of the Nankoweap overlies the Cardenas, and locally an angular discordance can be recognized that reflects the truncation of 60 m of Cardenas. This unconformity also underlies a newly recognized ferruginous sandstone of probable local extent that underlies the upper member and that herein is called the ferruginous member of the Nankoweap. Truncation of the Cardenas beneath the ferruginous and upper members locally may have been as much as 300 m. Basal conglomeratic sandstone of the upper member locally overlies the ferruginous member with apparent disconformity, reflecting a probable hiatus in deposition. Stratigraphic and structural relationships indicate that a ferruginous weathered zone was developed on an erosionally truncated section of the Cardenas before, and perhaps during, the time of deposition of the ferruginous member. Erosion of the ferruginous weathered zone provided material for the ferruginous member of the Nankoweap. The ferruginous weathered zone locally was faulted against unweathered Cardenas before deposition of the upper member of the Nankoweap. The Nankoweap Formation is disconformably overlain by marine strata of the Chuar Group. Three distinct units separated by unconformities (the Unkar Group, Nankoweap Formation, and Chuar Group) thus are recognized in the Grand Canyon Series of Walcott. Following current Stratigraphic practice, the Grand Canyon Series is herein redesignated the Grand Canyon Supergroup.

High Resolution Polarity Records and the Stratigraphic and Magnetostratigraphic Correlation of Late Miocene and Pliocene (Pannonian, s.l.) Deposits of Hungary

Stratigraphic records from four widely spaced holes, continuously cored from the surface of the Great Hungarian Plain to depths of 1.2 to 2 km, have been correlated and placed in relative stratigraphie positions by means of seismic profiles. Polarity zonations for two 2-km-thick cored sections were correlated with the polarity time scale for much of late Miocene time, and two cored sections, 1.2 km thick, were correlated with the polarity time scale for much of Pliocene and Pleistocene time. The high-resolution magnetostratigraphic records from the four cored sections contain many more polarity reversals than the accepted polarity time scale for the late Miocene and Pliocene; because of this, only the broader polarity intervals in the drill cores are correlated with the polarity time scale. The new seismic-stratigraphic and magnetostratigraphic correlations have led to revised correlations of Pannonian stratigraphie units in the subsurface, and to a new chronostratigraphic framework and model for the manner and timing of accumulation of late Miocene and Pliocene deposits in the Pannonian Basin. The high resolution polarity zonations also provide new information on the detailed character of the polarity time scale for parts of late Miocene, Pliocene, and Pleistocene time.

Correlation of Seismo- and Magnetostratigraphy in Southeastern Hungary

Correlation of results from magnetostratigraphic and seismic-reflection studies indicate that the Pannonian Basin, during the postrift phase of its evolution (middle Miocene to present), became filled by sediments of southward and eastward prograding deltaic wedges.

Stratigraphy and magnetic polarity of the high terrace remnants in the upper Ohio and Monongahela Rivers in West Virginia, Pennsylvania, and Ohio☆

Abstract A synthesis of previous work and new data on the stratigraphy of high terraces of the Ohio and Monongahela Rivers upstream from Parkersburg, West Virginia, indicates a correspondence between terrace histories in the ancient Teays and Pittsburgh drainage basins. Four terraces are identified in each. Sediments of the lower three alluvial and slackwater terraces, correlated with Illinoian, early Wisconsin, and late Wisconsin glacial deposits, have been traced along the modern Ohio River through the former divide between the Teays and Pittsburgh basins. Sediments in the fourth terrace, the highest well-defined terrace in each basin, were deposited in two ice-dammed lakes, separated by a divide near New Martinsville, West Virginia. Some deposits of the highest slackwater terrace in both the Teays and Pittsburgh basins have reversed remanent magnetic polarity. This, and the stratigraphic succession in the two basins, suggests that both were ponded during the same glaciation. Reversed polarity in these terrace sediments restricts the age of the first ice-damming event for which stratigraphic evidence is well-preserved to a pre-Illinoian, early Pleistocene glaciation prior to 788,000 yr ago. In contrast, slackwater sediments in the Monongahela River valley, upstream from an outwash gravel dam at the Allegheny-Monongahela confluence, have normal remanent magnetic polarity, corroborating correlation with an Illinoian ponding event.

Low- to high-amplitude oscillations and secular variation in a 1.2 km late Miocene inclination record

Abstract A Miocene section of 2 km thickness was continuously cored near Szombathely, NW Hungary. A detailed magnetostratigraphic study has been integrated with results of lithologic, sedimentologic, and paleontologic studies. Progressive alternating field (a.f.) and thermal demagnetization, and rock magnetic and mineralogic studies indicate that the natural remanent magnetization (NRM) resides in magnetite and the strata contain only minor secondary magnetizations. Major polarity zones have been defined by inclinations and correlated with the geomagnetic polarity time scale for the interval 10-9 Ma. Inclinations for samples collected at 1 2 m intervals display fine-scale oscillations representing secular variation with period times of 400–700 years. Fluctuating amplitudes of oscillation range from low (less than 10° peak-to-peak) to high (more than 40°), reflecting apparently varying stabilities of the geomagnetic field. Additionally, amplitudes of oscillation progress from low to high, and return to low, forming oscillation cycles with a periodicity of 6.2 ± 1.8 kyr. The oscillations and boundaries of oscillation cycles are generally unrelated to lithology and stratigraphy. Many oscillation cycles appear to be incomplete and ‘interrupted’ by high-level oscillations. Some incomplete cycles appear to arise from brief interruptions to the depositional record, whereas other interruptions may irregularly arise from a separate component of the geomagnetic field.