Bruin Bay Fault, Contact Point

Bruin Bay Fault, Contact Point

by | Feb 15, 2022

Contact Point is a headland forming the southern shore of Bruin Bay on the western side of Cook Inlet, about 123 miles (198 km) northwest of Kodiak and 93 miles (150 km) west-southwest of Homer, Alaska. It was named in 1926 by George C. Martin of the U.S. Geological Survey. A geological contact is a boundary that separates one rock body from another and can be formed during sediment deposition, by the intrusion of magma, or through faulting or other deformation of rock beds that brings distinct rock bodies into contact. The Bruin Bay fault is a complex of many steeply dipping faults that extends roughly 308 miles (498 km) along the western Gulf of Alaska from Becharof Lake on the Alaska Peninsula, along the west coast of the Cook Inlet Basin, and north to the Castle Mountain fault system. The Bruin Bay fault is exposed about 4.5 miles (7 km) southwest of Contact Point where the fault separates rocks of the Alaska Peninsula terrane into a western volcanic rock-dominated block and an eastern sedimentary rock-dominated block. The western volcanic block served as the source rock for the eastern sedimentary rock during Jurassic and Cretaceous times. The rocks on the western side of the contact are of the Talkeetna Formation which consists of granodiorite and quartz monzonite from the Early and Middle Jurassic, and volcanic breccia and tuff, all somewhat altered or metamorphosed. Rocks on the eastern side of the contact are of the Naknek Formation from the Late Jurrasic which consists of siltstone, sandstone, and conglomerate. The combined thickness of the sedimentary layers exceeds 9,843 feet (3,000 m), though the average thickness of the formation is more typically 5,577-6,562 feet (1,700-2,000 m). Plate tectonics has played a major role in the formation of these rocks and evidence from paleomagnetism, fossil assemblages, and depositional environments suggests that the Peninsular terrane rocks formed originally in tropical waters and were transported across the proto-Pacific Ocean to their present location by the end of the Mesozoic. The deformation and uplift of these rocks are ongoing today. Differential rates of weathering and erosion are evident for the hanging rock to the west and footwall rocks to the east, as well as two volcanic dike intrusions of silicate-rich minerals that clearly delineate the fault plane. Megafossils, particularly bivalves and ammonites, are useful in distinguishing rocks that appear similar.

The rocks exposed on the western shore of Cook Inlet are well known for their extremely prolific fossilized marine invertebrates. Knowledge of the fossils extends back to the final days of Russian America when a significant collection was made by the Russian mining engineer Peter Doroschin and sent to the Russian capital at Saint Petersburg, where they were ultimately studied and described in 1871 by Karl Eduard von Eichwald who was a famous paleontologist. He was of German ancestry and was born in present-day Latvia, but spent much of his later career in Saint Petersburg, and most of his scientific articles were written in German, but appeared in various Russian publications. He found that the most common megafossils found in Middle Jurassic rocks from Cook Inlet are bivalves of the genus Retroceramus. Among the earliest named species of the genus were four established by Eichwald from western Cook Inlet which are R. porrectus, R. ambiguus, R. eximius, and R. lucifer. Bivalves are aquatic molluscs also known as pelecypods, living at all depths of the sea and in brackish and freshwater. There are about 25,000 living species and have the greatest biomass of any mollusc, and certain bivalve species are numerically dominant in many benthic ecosystems. The most primitive bivalves are infaunal, burrowing into soft sediments. Bivalves are well represented in the fossil record from the early Paleozoic because of their calcareous shells. The shell consists of two valves with a noncalcified connecting ligament holding the valves together at a hinge plate. Hinge ligament tension holds the valves in a gaping position, with valve closure affected by adductor muscles. The Middle Jurassic rocks at Contact Point are on the west side of the Bruin Bay fault and contain a rich diversity of marine invertebrate fossils indicating that the rocks were formed in relatively warm waters. The Upper Jurassic rocks of the Naknek Formation on the east side of the fault are represented by cold water invertebrates predominantly bivalves of the genus Buchia. This change in biodiversity from the Middle to Late Jurassic also occurs in several other places on the North American continent and it is not clear what caused the change in fossil diversity, but probably resulted from the northward movement of tectonic plates.

The ammonites of Cook Inlet were thoroughly studied by the paleontologist Ralph W. Imlay of the U.S. Geological Survey. Ammonites are a group of extinct marine molluscs closely related to living octopuses, squid, and cuttlefish, but not shelled nautiloids. Their fossil shells usually take the form of planispirals, like many snails, although non-spiraled forms have also been found. Because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are very rarely preserved in any detail. Many ammonites probably lived in the open water of ancient seas, rather than at the sea bottom, because their fossils are often found in rocks laid down under conditions where no bottom-dwelling life is found. They may have avoided predation by squirting ink, much like modern cephalopods, and ink is occasionally preserved in fossil specimens. The soft body of the creature occupied the largest segments of the shell at the open end of the coil. The smaller earlier segments were walled off and the animal could maintain its buoyancy by filling them with gas. Thus, the smaller sections of the coil would have floated above the larger sections. Few of the ammonites occurring in the Early and Middle Jurassic period reached a size exceeding 9.1 inches (23 cm) in diameter. The largest ammonites reached a diameter of 6.6 feet (2 m). The largest documented North American ammonite is from the Cretaceous, with specimens measuring 4.5 feet (137 cm) in diameter. Ammonites were extremely abundant especially during the Mesozoic era. Many genera evolved and ran their course quickly, becoming extinct in a few million years. Due to their rapid evolution and widespread distribution, ammonites are used by geologists and paleontologists for biostratigraphy. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geologic periods. Ammonites as a group survived through several major extinction events, although often only a few species. Each time, however, this handful of species diversified into a multitude of forms. Ammonite fossils became less abundant during the latter part of the Mesozoic, and their extinction, along with other marine animals and non-avian dinosaurs, has been attributed to the Cretaceous-Paleogene extinction event, marking the end of the Cretaceous Period. Read more here and here. Explore more of the Bruin Bay fault and Contact Point here:

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This ‘warming stripe’ graphic is a visual representation of the change in global temperature from 1850 (top) to 2021 (bottom). Each stripe represents the average global temperature for one year. The average temperature from 1971-2000 is set as the boundary between blue and red. The color scale goes from -0.7°C to +0.7°C. The data are from the UK Met Office HadCRUT4.6 dataset. 

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