Absaroka sequence
Encyclopedia
The Absaroka sequence was a cratonic sequence
that extended from the end of the Mississippian through the Permian
periods. It is the unconformity between this sequence and the preceding Kaskaskia
that divides the Carboniferous into the Mississippian and Pennsylvanian
periods in North America
.
Like the Kaskaskia sequence, Absaroka sedimentary deposits were dominated by detrital
or siliclastic rocks. The first sediments were deposited near the continent
al margins, particularly near the Ouachita
and Appalachian highlands. Characteristic of the strata from this time are cyclothems
: alternating marine
and non-marine strata indicative of changes in sea-level, probably due to cyclic glaciation in the Southern Hemisphere
.
In North America, the waters of the Absaroka sequence regressed westward as the highlands to the east steadily eroded. Restricted oceanic circulation in the west led to extensive evaporite
formation. By the end of the period, the regression was complete, and the marine strata were superseded by extensive red bed deposition.
change have been divided into at least six magnitudes of order. Each order is a span of years that become smaller. The smallest unit is described as a punctuated aggradational cycle (PAC) and represents between 225,000 and 100,000 years of sediment accumulation. In the field, these units are usually one to five meters thick and contain several different rock units. These units show quick changes in sea level that were controlled by climate change due to glaciers.
The fifth order is often called a cyclothem
and may contain several PAC sequences and generally represent about 500,000 years. Again, glaciation seems to be the cause of the cyclic nature of the strata. The first four orders, though are controlled more by continental shifts and orogenic cycles. The first order cycle being the Absaroka, the second being the Carboniferous Period, and third order being rock formations or groups which are generally 8 to 10 million years long. The fourth order might be controlled by large scale glaciation and global ocean patterns over the course of a million years.
Cratonic sequence
A cratonic sequence refers to a very large-scale lithostratographic sequence that covers a complete marine transgressive-regressive cycle across a craton...
that extended from the end of the Mississippian through the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
periods. It is the unconformity between this sequence and the preceding Kaskaskia
Kaskaskia sequence
The Kaskaskia sequence was a cratonic sequence that began in the mid-Devonian, peaked early in the Mississippian, and ended by mid-Mississippian time...
that divides the Carboniferous into the Mississippian and Pennsylvanian
Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
periods in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
.
Like the Kaskaskia sequence, Absaroka sedimentary deposits were dominated by detrital
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
or siliclastic rocks. The first sediments were deposited near the continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
al margins, particularly near the Ouachita
Ouachita
Ouachita may refer to:In geography:* Ouachita Mountains* Ouachita orogeny the geologic event that raised the Ouachita Mountains* Ouachita River* Ouachita Parish, Louisiana* Ouachita County, Arkansas* Ouachita National Forest...
and Appalachian highlands. Characteristic of the strata from this time are cyclothems
Cyclothems
In geology, cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, sometimes interbedded with coal seams. Historically, the term was defined by the European coal geologists that worked in coal basins formed during the Carboniferous and earliest Permian periods...
: alternating marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
and non-marine strata indicative of changes in sea-level, probably due to cyclic glaciation in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
.
In North America, the waters of the Absaroka sequence regressed westward as the highlands to the east steadily eroded. Restricted oceanic circulation in the west led to extensive evaporite
Evaporite
Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...
formation. By the end of the period, the regression was complete, and the marine strata were superseded by extensive red bed deposition.
Transgressive–regressive units
These cycles of sea levelSea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
change have been divided into at least six magnitudes of order. Each order is a span of years that become smaller. The smallest unit is described as a punctuated aggradational cycle (PAC) and represents between 225,000 and 100,000 years of sediment accumulation. In the field, these units are usually one to five meters thick and contain several different rock units. These units show quick changes in sea level that were controlled by climate change due to glaciers.
The fifth order is often called a cyclothem
Cyclothems
In geology, cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, sometimes interbedded with coal seams. Historically, the term was defined by the European coal geologists that worked in coal basins formed during the Carboniferous and earliest Permian periods...
and may contain several PAC sequences and generally represent about 500,000 years. Again, glaciation seems to be the cause of the cyclic nature of the strata. The first four orders, though are controlled more by continental shifts and orogenic cycles. The first order cycle being the Absaroka, the second being the Carboniferous Period, and third order being rock formations or groups which are generally 8 to 10 million years long. The fourth order might be controlled by large scale glaciation and global ocean patterns over the course of a million years.