Rocky Cove, Cook Inlet

Rocky Cove, Cook Inlet

by | Feb 25, 2022

Rocky Cove is an embayment about 1 mile (1.6 km) wide at the base of Step Mountain, between Ursus Cove to the north and Bruin Bay to the south on the western shore of Cook Inlet, about 128 miles (206 km) northeast of Naknek and 76 miles (122 km) west-southwest of Homer, Alaska. The name was first published in 1903 by George C. Martin, a petroleum geologist working for the U.S. Geological Survey. Step Mountain forms the headland called Tignagvik Point on the south side of Rocky Cove. Cook Inlet represents a large basin of Tertiary geological time scale rock lying between two accreted belts of older Mesozoic rocks along the southern Alaska coast. Partially filling the basin and exposed on the east and west sides of Cook Inlet is a sequence of Tertiary sedimentary rocks as much as 22,966 feet (7,000 m) thick. These Tertiary rocks are important oil and gas reservoirs for petroleum thought to be sourced from underlying Jurassic and Triassic rocks. Bounding the western side of the basin are the Mesozoic and Cenozoic plutonic rocks of the Alaska-Aleutian Range batholith, along with associated volcanic rocks of the Early Jurassic Talkeetna Formation, and sedimentary rocks largely of Jurassic age derived from erosion of the Talkeetna magmatic arc and the batholith. The Bruin Bay Fault system runs along the coast of Rocky Cove and cuts the rocks of the Alaska Peninsula into a western pluton-dominated block and an eastern sedimentary rock-dominated block. The western plutonic rock was eroded and served as the source material for the eastern sedimentary rock during the Jurassic and Cretaceous times. The eastern sedimentary rock is called the Naknek Formation that consists of over 9,843 feet (3,000 m) of sandstone, conglomerate, and siltstone. The rocks at Rocky Cove are part of the Naknek Formation and consist of dark-gray to black siltstone and calcareous gray sandstone that was deposited in moderately deep water, well below wave base and above carbonate compensation depth, in a basin having restricted ocean circulation. The main fossils in the rock are the pelecypod Buchia and ammonites Amoeboceras, Phylloceras, and Perisphinctes. Sedimentary deposits record a history dominated by repeated glaciation during the Tertiary as well as periods of deposition in estuarine, lacustrine, and alluvial environments. Unconsolidated deposits at the surface were mainly derived from retreating glaciers that had advanced into the basin from surrounding mountains in the late Pleistocene, perhaps at times filling the basin with ice. During the glacial retreat, a saltwater incursion into the basin as sea level rose formed an ancestral Cook Inlet estuary.

The entrance to Rocky Cove is obstructed by reefs that extend about 2 miles (3.2 km) offshore and are exposed at low tides. The sea cliffs, reefs, and intertidal wave-cut platforms of Rocky Cove are erosional features that indicate an actively retreating coast. Cliff erosion or retreat depends on many factors including the structure of the rock and whether it is igneous, metamorphic, or sedimentary; the rock permeability, solubility, hardness, and jointing; the exposure to waves, tides, and currents; and subaerial weathering by surface and subsurface runoff, freeze-thaw cycles, and chemical processes. Hard rock cliffs such as basalt or metamorphics may retreat less than 0.4 inches (1 cm) per year, whereas soft sedimentary rock cliffs such as siltstone and sandstone may retreat more than 3.3 feet (1 m) per year. Cliff retreat is often episodic, with a cliff face collapsing following a period of undercutting of the cliff base by wave quarrying, abrasion, and weathering. Wave quarrying is the process of rock disintegration by breaking waves which produce shock pressures and air compression in small joints and fissures. Wave abrasion is the process of rock scraping and shattering by abrasive material such as rock fragments that move back and forth with the wave-orbital motion. Rock weathering is due to the mechanical action of alternate drying and wetting and salt crystallization in fissures, but also due to chemical processes such as dilution and biotic activity. Wave-cut platforms form when destructive waves hit against the cliff face, causing an undercut between the high and low watermarks, ultimately creating a wave-cut notch. This notch then enlarges as waves continue to undermine the cliff face until the roof of the notch can no longer hold the weight and it collapses resulting in the cliff retreating landward. The base of the notch forms the floor of the wave-cut platform as the collapsed material is broken into smaller particles and transported away. The uplands adjacent to the sea cliff at Rocky Cove, including Step Mountain, represent a sequence of ‘steps’ which are raised platforms or marine terraces resulting from a combination of tectonic coastal uplift and Quaternary sea-level fluctuations. The west side of Cook Inlet is generally being uplifted tectonically and by post-glacial rebound so the marine terraces are clearly evident, even though seismic events such as the 1964 Alaska earthquake caused widespread subsidence.

A wave-cut platform represents an extremely hostile biological environment because the continual wave action and active sediment transport by waves and currents along the base of sea cliffs and on shallow subtidal rock platforms may prevent the recruitment and colonization of benthic marine plants and animals. Some marine plants and animals have adapted to this environment and may colonize the rock platforms and reefs that are only exposed during the lowest tides. Aerial mapping of intertidal biota at Rocky Cove inferred a nearly continuous band of brown alga called the winged kelp, or Alaria marginata. Kelps are large brown algae or seaweed of which there are about 30 different genera. Algae consist of several groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom. The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, green algae, red algae, and brown algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, and they are no longer classified as plants. Kelps often grow in dense aggregations or kelp forests in shallow oceans and are thought to have appeared in the Miocene time, about 23 to 5 million years ago. They are known for their high growth rate, and some such as the giant kelp Macrocystis and the bull kelp Nereocystis can grow as fast as 1.6 feet (0.5 m) per day, and reach lengths of 100-260 feet (30-80 m). Alaria marginata can grow up to 13 feet (4 m) with long narrow fronds that have a distinctive raised midrib and wavy edges. This kelp is known to occupy wave-exposed intertidal habitats in the northeastern Pacific from Alaska to near Point Conception, California, and commonly grows in large patches that provide habitat for many other intertidal organisms. Read more here and here. Explore more of Rocky Cove here:

About the background graphic

This ‘warming stripe’ graphic is a visual representation of the change in global temperature from 1850 (top) to 2022 (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. 

Credit: Professor Ed Hawkins (University of Reading). Click here for more information about the #warmingstripes.

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