Rodger E. Denison

Transcontinental Proterozoic provinces

Research on the Precambrian basement of North America over the past two decades has shown that Archean and earliest Proterozoic evolution culminated in suturing of Archean cratonic elements and pre-1.80-Ga Proterozoic terranes to form the Canadian Shield at about 1.80 Ga (Hoffman, 1988,1989a, b). We will refer to this part of Laurentia as the Hudsonian craton (Fig. 1) because it was fused together about 1.80 to 1.85 Ga during the Trans-Hudson and Penokean orogenies (Hoffman, 1988). The Hudsonian craton, including its extensions into the United States (Chapters 2 and 3, this volume), formed the foreland against which 1.8- to 1.6-Ga continental growth occurred, forming the larger Laurentia (Hoffman, 1989a, b). Geologic and geochronologic studies over the past three decades have shown that most of the Precambrian in the United States south of the Hudsonian craton and west of the Grenville province (Chapter 5) consists of a broad northeast to east-northeast-trending zone of orogenic provinces that formed between 1.8 and 1.6 Ga. This zone, including extensions into eastern Canada, comprises or hosts most rock units of this age in North America as well as extensive suites of 1.35- to 1.50-Ga granite and rhyolite. This addition to the Hudsonian Craton is referred to in this chapter as the Transcontinental Proterozoic provinces (Fig. 1); the plural form is used to denote the composite nature of this broad region. The Transcontinental Proterozoic provinces consist of many distinct lithotectonic entities that can be defined on the basis of regional lithology, regional structure, U-Pb ages from zircons, Sr-Nd-Pb isotopic signatures, and regional geophysical anomalies.

Using Strontium Isotopes to Determine the Age and Origin of Gypsum and Anhydrite Beds

The variation of 87Sr/86Sr in seawater with time can be used to determine the age of calcium sulfate beds precipitated from brine derived from seawater. The determination of a marine origin can be established by the consistency of strontium isotope results from samples at different stratigraphic levels in a single gypsum/anhydrite bed collected at separated sites. The scatter of strontium isotope results in the gypsum/anhydrite samples examined here is interpreted to result from the contribution of meteoric strontium to the salina. Strontium isotope results from three evaporite settings, the Jurassic Todilto Formation from New Mexico, the Permian Blaine Formation from Blaine County, Oklahoma, and the Permian Salado Formation from New Mexico, are used to determine the age and origin of the parent brine for these gypsum/anhydrite beds. The calcium sulfate beds from each of these formations precipitated from salinas that originated with a marine flooding but show, at different times and localities, volume dominance by meteoric water. The marine/meteoric mixing of ancient salinas can be modeled by using strontium concentrations and isotope ratios estimated from modern analogs. A limited comparison of strontium and sulfur isotopes shows that sulfur isotopes in the settings studied are less sensitive to meteoric influx.

Basement Rocks in Continental Interior of United States

This paper outlines the Precambrian geologic history of the continental interior of the United States by describing several selected areas from north to south. In this way the history is developed chronologically and the various areas illustrate the methods used in preparing a map of the buried basement. Lateral continental accretion is of lesser importance than is usually postulated; at least 50 percent of North America was in existence 2,500 m.y. ago. Crustal stabilization occurred at different times (about 2,500, 1,700, 1,350, and 1,000 m.y. ago) in the North American continental interior. Buried basement rocks from more than 3,000 wells and scattered outcrops in the continental interior of the United States were studied petrographically. This information, in conjunction with geophysical data and isotopic ages, was used to outline geological units and to work out the geological history. Except for the Cambrian igneous complex of the Wichita Mountains, southern Oklahoma, all basement rocks between the Rocky and Appalachian Mountains are Precambrian. Included here as basement are the widespread Precambrian sedimentary and volcanic rock sequences as well as the crystalline continental crust usually defined as basement. Rocks 2,500 m.y. old or older exposed in northern Minnesota occur as belts of west-southwest-trending granite and gneiss alternating with greenstone and schist. They extend without interruption into eastern North Dakota and northeastern South Dakota. A large part of Wyoming also is underlain by rocks at least 2,500 m.y. old. Well data for the Williston basin in eastern Montana and western North and South Dakota are too sparse to permit construction of a geologic map of the basement but isotopic ages and gravity anomalies suggest that this region is underlain by rocks deformed and consolidated about 1,700 m.y. ago during the Black Hills orogeny. The sedimentary parent rocks of the Precambrian rocks exposed in the Black Hills were deposited on an older basement that contains zircon dated at 2,550 m.y. The Black Hills are the culmination of the long north-northwest-trending basement high that includes the Chadron and Cambridge arches of Nebraska and the Central Kansas uplift. Most wells to basement have penetrated metamorphic rocks along this uplift. A granite batholith underlies the End_Page 2351------------------------------ area along the Nebraska-Kansas boundary. North of this batholith, along the uplift, isotopic ages of granite and gneiss are similar to those from the Black Hills (ca. 1,700 m.y.). The batholith intrudes mafic schist as well as the granite and gneiss; Rb-Sr ages are 1,490 m.y.; K-Ar ages of biotite from the granite batholith and schist as well as whole-rock ages of the schist are between 1,360 and 1,300 m.y. These 1,360-1,300-m.y. ages reflect a period of metamorphism or thermal activity subsequent to the original formation of the rocks but are not known to be associated with a granite-forming event here. K-Ar ages of biotite from cataclastic zones in southwestern Nebraska are 1,170 m.y. A body of anorthosite-gabbro in southwestern Nebraska has not been dated, but its cataclastic textu e and a zone of cataclasis that trends toward the body suggest that its intrusion is either contemporaneous with, or older than, the time of cataclasis. Isotopic ages in central Kansas suggest two thermal events: 1,450-1,350 m.y. and 1,250-1,100 m.y. The latter is defined on the basis of Rb-Sr and K-Ar mica ages, the former on Rb-Sr feldspar or whole-rock analyses. Granite in southeastern Nebraska, northeastern Kansas, and central Missouri is at least 1,450 m.y. old. Most of the Nemaha uplift as well as many granites at least as far west as south-central New Mexico give isotopic ages in the 1,450 to 1,350-m.y. range. During this period (the Nemaha igneous activity), rocks from New Mexico eastward to Ohio were consolidated to form the basement for younger extensive volcanic-intrusive complexes. Keweenawan basalt and associated sedimentary rock units are nearly continuous from northeastern Kansas to the outcrop belt around Lake Superior. An offset in trend and a break in continuity near the Nebraska-Kansas border have been postulated to be the result of a post-Precambrian transcontinental wrench fault. Cataclasis, recognizable in some basement samples within this discontinuity, could be associated with either rifting of the Keweenawan trough or Paleozoic movement along the adjacent north-trending Nemaha uplift. Extensive areas in southeastern Missouri, northeastern Oklahoma and vicinity, and in and near the Texas Panhandle are underlain by rhyolite and related granite. Rhyolite is found in isolated localities in Wisconsin and in some of the few basement tests in Illinois, Indiana, and Ohio. Three distinct volcanic-intrusive events can be defined: St. Francois igneous activity of southeastern Missouri (to Ohio?), 1,350-1,200 m.y. ago; Spavinaw igneous activity of northeastern Oklahoma, 1,300-1,150 m.y. ago (with a 1,200 ± 30 m.y. isochron); and Panhandle igneous activity of Texas and eastern New Mexico, 1,200-1,100 m.y. ago (1,140 ± 50 m.y. average). Each igneous event produced micrographic granite as well as rhyolite (many samples show well-preserved welded tuff textures). Rhyolite from the Spavinaw and Panhandle igneous activities covers at least 24,000 and 21,500 sq mi, respectively; the original extent of the volcanic fields is unknown. Exposed in the Llano uplift of central Texas is a thick sequence of sedimentary rocks that were deposited, folded, metamorphosed, and intruded 1,150 to 1,000 m.y. ago during the Llano orogeny. Rocks associated in time with this event are found in the subsurface of north-central Texas and Trans-Pecos Texas. Extensive areas are covered by sedimentary rocks of probable or known Precambrian age. From north to south these include: Sioux Quartzite, minimum age 1,200 m.y.; upper Keweenawan sediments in the Keweenawan trough; sediments of southwestern Missouri-southeastern Kansas; Rice Formation of Scott, central Kansas; Tillman metasedimentary group of Ham et al., southern Oklahoma and adjacent parts of Texas; De Baca terrane of this report, south-central New Mexico (possibly correlative with the Swisher diabasic terrane = Swisher gabbroic terrane of Flawn, of the Texas Panhandle); and Fisher metasedimentary terrane of Flawn, north-central Texas (probably correlative with the Llano uplift metasedimentary rocks). The last major igneous event of the central United States before the Paleozoic marine inundation was the intrusion and extrusion of the early to mid-Cambrian basalt-gabbro-rhyolite-granite igneous complex of the Wichita Mountains of southern Oklahoma. Known Precambrian rocks have yet to be penetrated by wells in the Gulf coastal plain. Crystalline basement rocks in the inner coastal plain appear to be metamorphosed Paleozoic rocks of the Ouachita system. Southward the basement is covered by thick sedimentary rocks of the Gulf Coast geosyncline.

Parent Brine of the Castile Evaporites (Upper Permian), Texas and New Mexico

ABSTRACT The Upper Permian (lower Ochoan) Castile Formation is a major evaporite sequence (10,000 km3) of calcite, anhydrite, and halite in west Texas and southeastern New Mexico. Traditionally the Castile brine has been considered to have been derived from seawater. This tradition has recently been challenged by two versions of the closed-basin drawdown model. They call for deposition from a mixed brine, in part marine and in large part nonmarine. They propose drawdown of as much as 500 m to form a major sink for ground water issuing from the surrounding Capitan reef complex. A large fraction of the solute in the brine body is inferred to have been recycled from older Permian evaporites on the surrounding shelf. Strontium-isotope analyses show no evidence that meteoric ground water was contributed to the Castile brine. From a stratigraphic, geographic, and lithologic array of 65 samples of anhydrite, gypsum, and calcite, 59 have an 87Sr/86Sr ratio of 0.706923 (sw of -225.0), a ratio that is the same as that of strontium in early Ochoan ocean water. If considerable (>15%) influx of meteoric water had occurred, enough continental strontium would have been introduced to have resulted in higher ratios. Low bromide values (20-40 ppm) in Castile halite, which have been used to argue for meteoric influx and for recycled salt, probably resulted from diagenesis. During shallow burial by halite, centimeter-size, bottom-grown crystals of gypsum were altered to nodular anhydrite. The rising water of dehydration caused the halite to recrystallize. During the recrystallization, some bromide was expelled. Despite the large volume of water that evaporated annually from its surface (52 km3/yr, assuming an evaporation rate of 2 m/yr), the Castile brine body never completely desiccated. The surrounding shelf was flat, hot, and generally dry. It probably could not have supplied a significant volume of meteoric spring water to the basin over tens of thousands of years. More likely, during the entire history of the evaporite sequence, influx was dominantly marine. Marine ground water flowed through the Capitan Formation into the evaporite basin along its southern and possibly western margin probably with a rate of flow that was usually fast enough to prevent major drawdown of the brine surface.

Variation in 87 Sr / 86 Sr of Permian Seawater: An Overview

The strontium isotope ratio (87Sr/S6Sr) of seawater has changed significantly through time in response to variations in the input of Sr to the oceans from various crust and upper mantle sources with differing 87Sr /86Sr values. An important aspect of this temporal variation is the empirical finding that, while the 87Sr /86Sr of seawater may vary through time, at any given time the Sr isotope ratio of the oceans is everywhere the same. Isotopic uniformity is achieved because the marine residence time of Sr (~ 4 X 106 yr) is long (e.g., Veizer 1989) compared to the oceanic circulation time (~ 103 yr). The oceans, therefore, are well mixed with respect to both Sr and Sr isotope ratio, and all coeval, marine-derived, carbonates, sulfates, and phosphates will possess the same initial 87Sr/S6Sr value. Consequently, the temporal changes in the 87Sr/S6Sr of seawater can be used to correlate and date marine strata and can provide insight into the global processes that have shaped our world.

Isotopic and elemental chemistry of subsurface Precambrian igneous rocks, west Texas and eastern New Mexico

We present major element, trace element, and Nd isotopic analyses from cuttings and core samples for three subsurface terranes in west Texas and eastern New Mexico. The most northerly is the Panhandle volcanic terrane, which represents a large part of the Mesoproterozoic southern granite-rhyolite province. This terrane is comprised of undeformed rhyolite, ignimbritic tuff, granite, and diabase. The Panhandle terrane is split by the Debaca terrane, which consists of intercalated metasedimentary and metavolcanic rocks intruded by olivine gabbro, ferrogabbro, and diabase. Mildly to strongly deformed intermediate and felsic intrusive rocks of unknown affinity make up the third terrane, called here the crystalline terrane; it is located south and southeast of the Panhandle and Debaca terranes. Intermediate-to-felsic rocks of the terranes can be subdivided on the basis of their geochemistry into those with: (1) K 2 O/Na 2 O > 1 and A-type trace element characteristics; and (2) K 2 O/Na 2 O 1.7-Ga crust. The southern edge of Laurentia, therefore, is farther south than previously inferred. A diabase from the Panhandle terrane has a T DM of 1.44 Ga. If this model age is close to the crystallization age, then diabase in the Panhandle terrane is approximately coeval with the granite and rhyolite. The model age for the gabbro from the Debaca terrane is distinctly younger at 1.26 Ga, and is the same as crystallization ages of felsic tuffs associated with shelf carbonates in the Franklin Mountains and Van Horn area. In the crystalline terrane, both A- and I-type granites are present. Model ages for the I-type granites are 1.40–1.47 Ga. These are distinctly younger than the model age for the Panhandle terrane, and an A-type granite has a T DM of 1.35 Ga. These data indicate that granites in the crystalline terrane are not part of the granite-rhyolite province; rather, they constitute a separate group.

Geology of the Blue River Gneiss, Eastern Arbuckle Mountains, Oklahoma

The Blue River Gneiss, along with the Burch Granodiorite, Troy Granite, and Tishomingo Granite compose the Proterozoic basement in the eastern Arbuckle Mountains of Oklahoma. The Blue River, in contrast to the other three units, shows the effects of a pervasive regional metamorphism and recrystallization of constituent minerals. This metamorphic overprint produced a foliation and locally a metamorphic layering in those rocks having a sufficiently high color index. In the more leucocratic phases, a mineral orientation defines the foliation. In other rocks, an otherwise homogeneous granitoid contains scattered K-feldspar megacrysts (porphyroblasts or relicts?) that display a preferred orientation.

Overview of the Central North American Basins and Their Relation to Deep Crustal Structure: ABSTRACT

As our knowledge of deep structure of major central North American basins has increased, it has become clear that they have experienced long and complicated tectonic histories. A knowledge of these histories is especially important to efforts to formulate exploration strategies for deeper horizons and frontier areas. Regional geophysical and geologic studies of these basins indicate that Precambrian features have often exerted considerable control on basinal development (e.g., Anadarko basin, Rome trough, Rough Creek graben, Pedregosa basin). A particularly important tectonic event was the Eocambrian continental breakup which extensively rifted the southern margin of North America. Although this rifting event is manifested in various ways, its extent can be estimated by m pping the deep-seated crustal anomalies which probably formed at this time. Although age relations are uncertain in most cases, deep-seated anomalies are associated with the Arkoma basin, Anadarko basin, Illinois basin, Mississippi embayment, and Permian basin. There are many similarities in the development of these basins, but they all can be shown to have unique tectonic histories. End_of_Article - Last_Page 495------------

Basement in the Continental Interior of the United States: ABSTRACT

Buried basement rocks of the central United States are mainly plutonic granitic rock, mafic and felsic metamorphic rock, and diabase. Rhyolite, granitic rock, and gabbro-diabase form a discontinuous belt from eastern New Mexico to eastern Missouri. The 0.5 b.y. rocks in the Wichita Mountains, Oklahoma, are the result of the last major igneous event. The Ouachita structural belt is the southern limit of petrographic knowledge of plutonic rocks. None of these rock groups bears a simple relationship to basement topography and isotopic age. The Black Hills uplift-Cambridge arch-Central Kansas uplift is underlain dominantly by metamorphic rock with ages ranging from 1.7 b.y. in the north to 1.2 b.y. in the south. Siouxia arch contains 1.4 b.y. granitic and metamorphic rock. Nemaha uplift is underlain by 1.2-1.5 b.y. granite. Diverse rock types of 1.2-1.4 b.y. underlie Amarillo uplift and Red River-Matador arch. Several large gravity anomalies correspond to major basement structures. The Williston basin is bounded on the south and west by a series of major west- and northwest-trending gravity anomalies and on the east by a belt of south-trending gravity anomalies extending from Canada into the central Dakotas that coincides with the boundary between the 2.5 b.y. Superior and the 1.7 b.y. Churchill Provinces of the Canadian Shield. The Sioux formation (minimum age 1.2 b.y.) lies along the west-trending anomaly in southeastern and central South Dakota. Keweenawan basaltic and sedimentary rock coincides with the mid-continent gravity anomaly and extends nearly continuously from Lake Superior to northeastern Kansas. The prominent gravity feature along the Red River-Matador arch coincides with the boundary between 1.2-1.4 b.y. rocks in the central United States and the 1.0 b.y. rocks in Texas. End_of_Article - Last_Page 539------------