Omarolluk
Encyclopedia
Omarolluks, sometimes shortened to "omars," are a distinctive type of glacial erratic
that consists of dark siliceous greywacke
and exhibits prominent rounded, often deep, hemispherical voids and pits. The hemispherical voids and pits result from the selective dissolution of carbonate concretion
s within the greywacke. The greywacke, of which Omars consist, is identifiable by its low metamorphic grade and the 10%–40% rock fragments, distinctive volcanic clasts, and spherical carbonate concretions that it contains. Omars are typically rounded and range in size from pebbles to boulders. Their rounded shape, whether found in glacial tills
or glacial-fluvial (outwash) gravels, indicate that they were eroded from pre-existing littoral or fluvial
deposits. Omars are typically found associated with granule
s and pebble
s of oolitic
jasper
that were transported from the Belcher Islands
in Hudson Bay
, Canada
.
The name given these glacial erratics refer to their source, which is the Proterozoic
Omarolluk Formation in the Belcher Islands in southeast Hudson Bay. The Laurentide Ice Sheet eroded omars from the Belcher Islands, an archipelago
limited to only about a quarter of 1% of Hudson Bay. Glaciers moved omars from the southeastern part of Hudson Bay to central Canada and into the U.S. where they were deposited on moraines. Because scientists know precisely where they came from they are very valuable in documenting the movement of glaciers.
Glacial erratic
A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. "Erratics" take their name from the Latin word errare, and are carried by glacial ice, often over distances of hundreds of kilometres...
that consists of dark siliceous greywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
and exhibits prominent rounded, often deep, hemispherical voids and pits. The hemispherical voids and pits result from the selective dissolution of carbonate concretion
Concretion
A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity . Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'...
s within the greywacke. The greywacke, of which Omars consist, is identifiable by its low metamorphic grade and the 10%–40% rock fragments, distinctive volcanic clasts, and spherical carbonate concretions that it contains. Omars are typically rounded and range in size from pebbles to boulders. Their rounded shape, whether found in glacial tills
Till
thumb|right|Closeup of glacial till. Note that the larger grains in the till are completely surrounded by the matrix of finer material , and this characteristic, known as matrix support, is diagnostic of till....
or glacial-fluvial (outwash) gravels, indicate that they were eroded from pre-existing littoral or fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
deposits. Omars are typically found associated with granule
Granule (geology)
A granule is a clast of rock with a particle size of 2 to 4 millimetres based on the Krumbein phi scale of sedimentology. Granules are generally considered to be larger than sand and smaller than pebbles . A rock made predominantly of granules is termed a conglomerate....
s and pebble
Pebble
A pebble is a clast of rock with a particle size of 4 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered to be larger than granules and smaller than cobbles . A rock made predominantly of pebbles is termed a conglomerate...
s of oolitic
Oolite
Oolite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are called pisolites...
jasper
Jasper
Jasper, a form of chalcedony, is an opaque, impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. This mineral breaks with a smooth surface, and is used for ornamentation or as a gemstone. It can be highly polished and is used for vases, seals, and at one time for...
that were transported from the Belcher Islands
Belcher Islands
The Belcher Islands are an archipelago in Qikiqtaaluk Region, Nunavut, Canada. Located in Hudson Bay, the Belcher Islands are spread out over almost . The hamlet of Sanikiluaq is on the north coast of Flaherty Island and is the southernmost in Nunavut. Along with Flaherty Island, the other large...
in Hudson Bay
Hudson Bay
Hudson Bay , sometimes called Hudson's Bay, is a large body of saltwater in northeastern Canada. It drains a very large area, about , that includes parts of Ontario, Quebec, Saskatchewan, Alberta, most of Manitoba, southeastern Nunavut, as well as parts of North Dakota, South Dakota, Minnesota,...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
.
The name given these glacial erratics refer to their source, which is the Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
Omarolluk Formation in the Belcher Islands in southeast Hudson Bay. The Laurentide Ice Sheet eroded omars from the Belcher Islands, an archipelago
Archipelago
An archipelago , sometimes called an island group, is a chain or cluster of islands. The word archipelago is derived from the Greek ἄρχι- – arkhi- and πέλαγος – pélagos through the Italian arcipelago...
limited to only about a quarter of 1% of Hudson Bay. Glaciers moved omars from the southeastern part of Hudson Bay to central Canada and into the U.S. where they were deposited on moraines. Because scientists know precisely where they came from they are very valuable in documenting the movement of glaciers.