Richard W. Ojakangas

Cretaceous Sedimentation, Sacramento Valley, California

Upper Jurassic and Cretaceous sedimentary rocks more than 35,000 feet thick are exposed along the west side of the Sacramento Valley of California. To ascertain the framework in which the sediments were deposited, a detailed study of these rocks was made in the Cache Creek-Rumsey Hills area, and a general study was made at other localities. The sedimentary section consists of interbedded sandstones, which are commonly graded, mudstones, and siltstones, with minor conglomerates and bentonitic rocks. Units that are dominantly sandstone alternate with units that are dominantly mudstone. The sequence is sparsely fossilferous, but approximate stage boundaries were determined. Nine hundred paleocurrent measurements were made on sole marks, parting lineations, cross-bedding, and other structures. These indicate that paleocurrents in general flowed from north to south, parallel to the regional tectonic trend, the eastern shoreline of the basin of deposition, and the isopachous lines of the sequence. During Cenomanian and early Turonian time, however, currents moving toward the west were prominent. The mineralogy of the sandstones was determined through the Cache Creek-Rumsey Hills section, and check samples from elsewhere in the belt were found to be similar. The sandstones include feldspathic, arkosic, and lithic wackes and arenites, with the main grain constituents being quartz, feldspar, and volcanic rock fragments. The K-feldspar content is greatest in the Upper Cretaceous rocks. Epidote is the dominant non-opaque, non-micaceous heavy mineral; apatite, sphene, hornblende, and zircon are also common. Six intervals of differing composition were established. In Interval I (Upper Jurassic), andesitic (?) volcanic detritus is dominant in the few samples studied; Interval II (Valanginian) contains abundant quartz and plagioclase; in Interval III (Hauterivian through middle Albian), quartz is dominant; Interval IV (upper Albian and Cenomanian) is characterized by K-feldspar and andesitic (?) volcanic rock fragments; Interval V (Turonian to upper Santonian) contains K-feldspar, plagioclase, quartz, and acidic volcanic detritus; and Interval VI (upper Santonian and Campanian) is characterized by K-feldspar, plagioclase, and quartz. The clayey fractions of sandstones and associated mudstones are similar, with chlorite dominant through much of the Lower Cretaceous, and mica and montmorillonite dominant in the Upper Cretaceous. Major diagenetic changes in the sandstones include widespread carbonate cementation and replacement, recrystallization of matrix, and alteration of plagioclase and biotite. Albitization of plagioclase and chloritization of biotite were found to vary with depth and the content of calcite in the sandstones. These alterations proceeded to a small degree at depths of burial estimated to be as shallow as 10,000 feet, they affected all noncalcareous strata buried from 20 to 30,000 feet, and were extensively developed in even the calcareous rocks that were buried 35,000 feet. The ancestral Klamath Mountains and Sierra Nevada evidently were the sources of the detritus. Major events in the source areas, such as volcanism, pluton emplacement, and unroofing of plutons, apparently are reflected in the sedimentary rock column. The sequence was deposited below wave base, generally far from shore in an outer neritic-upper bathyal, probably miogeosynclinal, environment. Sedimentary structures and grading suggest deposition by turbidity currents. The sequence strikingly resembles other rock sequences which have been interpreted in the literature as “turbidites.” In an attempt to explain the sedimentation, three models are considered, each consisting of an elongated north-south basin open to the west: (1) a southerly paleoslope trending parallel to the long axis and the eastern shoreline of the basin, with currents carrying sediment southward down the paleoslope; (2) a paleoslope to the west from the north-south trending shoreline, with currents moving down the paleoslope and subsequently deflected to the south by deep oceanic currents; and (3) a subsea fan with an asymmetric cross section that caused the deflection of westerly moving turbidity currents toward the south. Modern counterparts of the last model are present off the western coast of North America. This model is tentatively suggested as the one which best fits the data.

Lower Proterozoic glaciogenic deposits, eastern Finland

In the lower Proterozoic Sariolian Group of eastern Finland, the metasedimentary Urkkavaara Formation possesses the attributes of a glaciogenic deposit. Within the formation, four members have been established—a lower siltstone-argillite member, a graded sandstone member, an upper siltstone-argillite member, and a diamictite member. The lowest three members contain lonestones. The upper siltstone-argillite member passes gradually over 1 m into a massive diamictite. Many lonestones in the siltstone-argillite members are clearly dropstones. These members, and their association with diamictite, constitute strong evidence for a glaciogenic origin for the formation. The vertical transition of siltstone-argillite into diamictite indicates a similar “raining-down” mechanism of deposition for the diamictite. The thickness of the formation, 30 to 60 m (minimum), suggests deposition from icebergs, rather than from seasonal ice, for the laminated members and the diamictite. The graded sandstone member is interpreted to be a result of turbidity current deposition near a glacial terminus. A glaciomarine environment is suggested. Lonestone-bearing units and diamictites in the Soviet Union just east of Finland have recently been reinterpreted as glaciogenic deposits. There appears, therefore, to have been an early Proterozoic continental glaciation about 2,500 to 2,300 m.y. ago on the Baltic Shield, approximately the same time that early Proterozoic glaciation occurred in North America.

Paleoproterozoic basin development and sedimentation in the Lake Superior region, North America

Abstract The peneplaned Archean craton in the Lake Superior region was the platform upon which a continental margin assemblage was deposited. Extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform. Seas transgressed onto the continent several times and an ocean basin opened south of the present-day Lake Superior. Island arcs that formed during subduction collided with the craton margin as the ocean basin closed; oceanic crust is poorly preserved as a dismembered ophiolite sequence. The arc volcanics are preserved as the Wisconsin magmatic terranes. The collision resulted in a fold-and-thrust belt known as the Penokean orogen. To the north of the fold-and-thrust belt, a northward-migrating foreland basin — the Animikie basin — developed. Thick turbidite successions were deposited along the basin axis, and terrigenous clastics and Lake Superior-type iron-formation were deposited on the shelf along the northern margin of the basin. The primary paleoclimatic indicators are: (1) glaciogenic rocks at the base of the Paleoproterozoic succession in Michigan indicating ice-house conditions; (2) remnants of a paleosol on the glaciogenic rocks indicative of deep weathering, probably under subtropical conditions and therefore of greenhouse conditions; and (3) carbonate minerals after gypsum, halite, and anhydrite in stromatolitic dolomite, indicative of aridity. Three second-order depositional sequences are bounded by major unconformities, and can be correlated throughout the Lake Superior region.

Burial Metamorphism of the Late Mesozoic Great Valley Sequence, Cache Creek, California

In sandstones of the Great Valley Sequence of Mesozoic age at Cache Creek, post-depositional albitization of plagioclase and chloritization of biotite are widespread in noncalcareous rocks, but uncommon in rocks with calcareous cement. The degree of alteration increases systematically with age and inferred depth of burial in Upper Cretaceous strata, and is uniformly great in Lower Cretaceous strata that were buried from 20,000 to 30,000 feet. Laumontite is characteristic of thoroughly altered Lower Cretaceous rocks. Other metamorphic assemblages may be widespread at higher and lower horizons.

Upper Precambrian (Eocambrian) Mineral Fork Tillite of Utah: A continental glacial and glaciomarine sequence

A glacial origin for the Mineral Fork Tillite in the Big Cottonwood Canyon area of the Wasatch Range in Utah is reaffirmed. The formation consists of two members: a lower sequence of thick, massive diamictites, with minor lensoid beds of shale-siltstone, sandstone, and conglomerate, and an upper sequence of more abundant shale-siltstone and siltstone that displays prominent bedding. The lower member is interpreted to be a sedimentary pile of tills and outwash deposited by multiple advances of continental glaciers. A grooved, polished, and striated surface marks the basal contact with the Big Cottonwood Formation. The upper member is considered to be of glaciomarine origin, mainly sediments released from icebergs and/or a floating ice shelf, along with rock flour washed from marine-based glacier margins. One major continental to marine cycle of environmental change is indicated as a result of slow but steady subsidence along a continental margin. Evidence cited here strengthens the case for a major glacial event during late Precambrian (Eocambrian) time in western North America.

Archean Volcanogenic Graywackes of the Vermilion District, Northeastern Minnesota

Archean graywackes of the Vermilion volcanic-sedimentary belt were derived in large part from felsic-intermediate volcanic accumulations (mostly dacitic) within the depositional basin. The graywackes are deficient in quartz and potash-feldspar, and are largely made up of volcanic rock fragments and plagioclase. Minor granitic detritus in sedimentary rocks at the eastern end of the belt can be traced to the Saganaga Granite which appears to have intruded the sequence during volcanism and sedimentation. Only fragmentary evidence for other granitic sources has been found. Grading is common in the graywackes, and other sedimentary structures typical of turbidite sequences are also visible locally despite metamorphism and deformation. Apparently turbidity currents moved volcanic detritus down the slopes of the felsic-intermediate volcanic piles into adjacent parts of the basin.

A possible southeastern extension of the Midcontinent Rift System located in Ohio

In 1988 a stratigraphic test core drilled in southwestern Ohio penetrated a previously unknown structure and sedimentologic unit. Core analyses disclosed a sequence of lithic arenite and siltstone of probable pre-Phanerozoic age, deposited within an alluvial-fluvial environment. A seismic reflection profile across the core site shows a sequence of strong, horizontal Paleozoic reflectors unconformably overlying eastward dipping, layered units of poor reflectivity. Beyond the core hole total depth, a regime of excellent reflectors is seen. The pre-Mount Simon (Upper Cambrian) sedimentary sequence encountered has been defined as the type section of the Middle Run Formation. Point counter analyses of typical Middle Run Formation samples indicate that this sequence is similar in composition to middle Proterozoic units associated with the Midcontinent Rift System in the Lake Superior region. The newly discovered structure in Ohio is also similar to half-graben structures composing the Lake Superior Basin portion of the Midcontinent rift. On the basis of similarities in lithology, stratigraphy, structure, and proximity to regional gravity anomalies, the Middle Run Formation and its apparent half-graben basin is proposed as evidence for the probable extension of the Midcontinent Rift System southward, from its normally accepted termination in southeastern Michigan, into southwestern Ohio.

The 1.1-Ga Midcontinent Rift System, central North America: sedimentology of two deep boreholes, Lake Superior region

Abstract The Midcontinent Rift System (MRS) of central North America is a 1.1-Ga, 2500-km long structural feature that has been interpreted as a triple-junction rift developed over a mantle plume. As much as 20 km of subaerial lava flows, mainly flood basalts, are overlain by as much as 10 km of sedimentary rocks that are mostly continental fluvial red beds. This rock sequence, known as the Keweenawan Supergroup, has been penetrated by a few deep boreholes in the search for petroleum. In this paper, two deep boreholes in the Upper Peninsula of Michigan are described in detail for the first time. Both the Amoco Production #1-29R test, herein referred to as the St. Amour well, and the nearby Hickey Creek well drilled by Cleveland Cliffs Mining Services, were 100% cored. The former is 7238 ft (2410 m) deep and the latter is 5345 ft (1780 m) deep. The entirety of the stratigraphic succession of the Hickey Creek core correlates very well with the upper portion of the St. Amour core, as determined by core description and point-counting of 43 thin sections selected out of 100 studied thin sections. Two Lower Paleozoic units and two Keweenawan red bed units—the Jacobsville Sandstone and the underlying Freda Sandstone—are described. The Jacobsville is largely a feldspatholithic sandstone and the Freda is largely a lithofeldspathic sandstone. Below the Freda, the remaining footage of the St. Amour core consists of a thick quartzose sandstone unit that overlies a heterogenous unit of intercalated red bed units of conglomerate, sandstone, siltstone, and shale; black shale; individual basalt flows; and a basal ignimbritic rhyolite. This lower portion of the St. Amour core presents an enigma, as it correlates very poorly with other key boreholes located to the west and southwest. While a black shale sequence is similar to the petroleum-bearing Nonesuch Formation farther west, there is no conglomerate unit to correlate with the Copper Harbor Conglomerate. Other key boreholes are distributed over a 1300-km distance along the better known southwest arm of the triple-junction MRS, and can be correlated rather well with the units that are exposed in the Lake Superior region. However, a definitive explanation of the anomalous, deeper St. Amour stratigraphy is elusive and any explanation is tenuous. A possible explanation for this anomalous stratigraphy may be the geographic proximity of the St. Amour borehole to the Keweenawan Hot Spot (mantle plume), the suggested thermal force behind the development of the MRS. Similarly, a drastic change in structural architecture may be explained by this geographic relationship. Thus, within the locale of this rifting center, complexities of expansion tectonics may well be responsible for igneous and sedimentary sequences that differ considerably from those found farther west along the rift arm.

Sedimentary Fill of the 1100 Ma (Keweenawan) Midcontinent Rift System in the Lake Superior Region

In the Lake Superior region, four “sequences” of sedimentary rocks reflect the tectonic-sedimentary framework immediately before, during, and after the 1100 Ma magmatic event that resulted in as much as 20,000 m of dominantly basaltic subaerial lava flows and large gabbroic intrusions (Behrendt et al. 1988).

Early Proterozoic Leptite and Halleflinta (Tuff and Tuffite) Sequences of Southern Finland Reinterpreted as Shear Zones: Significance to Lake Superior Geology

Many Early Proterozoic volcanogenic sequences in southern Finland and adjacent Sweden have long been interpreted as bedded tuffs and “tuffites” (i.e., epiclastic volcanic sediments), and commonly have been called leptites and halleflintas. While definitions of these unfamiliar terms vary, even in Fennoscandinavia, leptites and halleflintas can be described as fine-grained quartzofeldspathic metavolcanic rocks. The halleflintas are the finer grained of the two (<0.25mm) and are commonly thinly bedded (<2.0 cm). A restudy of several such units in Finland has provided evidence for a structural origin as tectonic layering (i.e., shear bands) rather than as primary bedding.