Arthur Grantz

Saline Springs, Copper River Lowland, Alaska

Saline waters reach the surface in a large area of the southeast Copper River Lowland, Alaska, and are estimated to add more than 200 tons of chloride per day to the Copper River. Most saline springs in the area are similar in composition to connate waters of the Na-Ca-Cl type, and are accompanied by gases composed mainly of methane and nitrogen. In the east part of the saline area three springs discharge Na-Cl-HCO3 type water that is thought to be basically water of the Na-Ca-Cl type admixed with water rich in HCO3, B, and SO4. These springs discharge CO2 gas. It is postulated that the Na-Cl-HCO3 waters are the result of mixing at depth of waters of the Na-Ca-Cl type with fluids of volcanic, metamorphic, or other ori in. Chemical data slightly favor a metamorphic origin for the admixing fluids. The saline water is thought probably to originate in Upper Cretaceous or older marine sedimentary rocks which are inferred to underlie the southern part of the Copper River Lowland.

Stratigraphic and structural significance of Cretaceous fossils from Tiglukpuk Formation, Northern Alaska

The presence of Buchia [= Aucella] sublaevis of Valanginian age at three localities within two units of different facies in the type Tiglukpuk Formation on Tiglukpuk Creek shows that most of this formation is of Early Cretaceous rather than of Jurassic age. The presence of Buchia okensis and B. subokensis of Early Cretaceous (Berriasian) age in the Okpikruak Formation shows that the Tiglukpuk Formation is younger than the Okpikruak Formation, which overlies it structurally. These age and structural relations suggest that thrust faults have juxtaposed different sequences of Buchia-bearing strata in the Tiglukpuk Creek and probably elsewhere in northern Alaska and that the Tiglukpuk Formation should be either greatly revised or abandoned.

Age of Arkose Ridge Formation, South-Central Alaska: GEOLOGICAL NOTES

The Arkose Ridge Formation of the southwestern Talkeetna Mountains adjacent to the lower Matanuska Valley, has been a stratigraphic enigma. The formation consists of arkose, shale, sandstone, and conglomerate, contains abundant plant material, and may be more than 6000 ft. thick. The fossil plants were first thought to be of Eocene Age; new collections indicate an Albian or possibly a Cenomanian Age for the formation. This age determination suggests that the Arkose Ridge Formation is correlative with mollusk-dated marine beds in the lower part of the Matanuska Formation; these marine strata vary throughout the region from a thin section of glauconitic calcareous sandstone, silty claystone, and algal limestone, to thin sections of feldspathic sandstone and siltstone.

Tectonic Implications of Some New Mesozoic Stratigraphic Data on Alaska: ABSTRACT

Increased geological exploration of Alaska during the past decade has produced a wealth of new stratigraphic data. Some of these data, particularly from Mesozoic rocks, are of fundamental importance in the interpretation of the tectonic history of Alaska. The U. S.:Geological Survey is compiling a comprehensive correlation chart of all known Mesozoic sedimentary, volcanic, and intrusive rocks. Preliminary work on the chart indicates the need for revising or refining present concepts of the stratigraphy and tectonics in several areas. For example, recent studies in northwestern Alaska suggest that the mid-Cretaceous Koyukuk geosyncline End_Page 274------------------------------ was not, as previously supposed, a single depositional trough stretching from the Brooks Range to the Yukon delta but consisted of a narrow east-trending trough along the Kobuk and upper Koyukuk Rivers and a larger southwest-trending trough that extended from the lower Koyukuk River to the Yukon delta. The two troughs were separated by an east-trending geanticline along Lat 66° N. The volcanics and granitic intrusives of Late Jurassic and Early Cretaceous age that are now exposed along this geanticlinal trend were an important source of the sediments in both troughs. Important data are also being developed by restudy of long-known Mesozoic terranes in southern Alaska. For example, the Talkeetna Formation of southeastern Talkeetna Mountains was thought to consist mainly of marine volcanics of Pliensbachian (Early Jurassic) age. However, it is now known to range in age at least from early Sinemurian to late Toarcian. Radioactivity dates indicate that, in the Talkeetna Mountains, plutons began to be intruded into the Talkeetna Formation shortly after it was deposited. Intrusion began at about the time the Matanuska epi-eugeosyncline was established in earliest Middle Jurassic time within the area of the more extensive eugeosyncline of the Talkeetna Formation. Intrusion probably continued into Late Jurassic and possibly Early Cretaceous time, although the adjacent Matanuska geosyncline was concurrently receiving a relatively complete sedimentary section. End_of_Article - Last_Page 275------------

Aeromagnetic Reconnaissance of Cook Inlet Area, Alaska: ABSTRACT

Fourteen aeromagnetic profiles were flown east-west across the Cook Inlet area in 1954, nine extending from about the Triumvirate and Capps glaciers to the Chugach Mountains, and five from End_Page 219------------------------------ the Iniskin-Chinitna Peninsula to the Kenai Peninsula. These profiles show several magnetic features that seem to have geologic significance. The over-all arched character of the profiles suggests a block-shaped rock mass underlying Cook inlet at great depth. A 1,600-gamma anomaly was observed over Mt. Susitna, a granitic intrusion. A two-dimensional anomaly observed over Knik Arm may reasonably be attributed to a zone of buried granitic intrusive rocks continuous with the intrusive cropping out at Eklutna. This intrusive, or zones of intrusives, appears to deepen southward, reaching estimated depths of 5,000-6,000 feet at the lower end of Knik Arm. Anomalies observed over the Susitna flats indicate that the magnetic basement is buried 12,000-14,000 feet. An abrupt magnetic rise of 300-400 gammas observed over the coast line of the Iniskin-Chinitna Peninsula is caused by a significant change in rock type, suggesting the possible existence of a fault with a vertical displacement of several thousand feet. East of this area, no near-surface anomaly-producing rock masses are present. It is likely that here the depth to magnetic basement is very great. End_of_Article - Last_Page 220------------