Dockum Group
Encyclopedia
The Dockum is a Late Triassic
(approximately late Carnian
through Rhaetian
, or 223-200 Ma) geologic group found primarily on the Llano Estacado
of western Texas
and eastern New Mexico
with minor exposures in southwestern Kansas
, eastern Colorado
, and Oklahoma
panhandle. The Dockum reaches a maximum thickness of slightly over 650 m but is usually much thinner. The Dockum rests on an unconformity
over the Anisian
(242-234 Ma) aged Anton Chico Formation. The Dockum and Chinle Formation
were deposited roughly at the same time and share many of the same vertebrate
s and plant
fossils. They appear to have very similar paleoenviroment. The two units are approximately separated by the Rio Grande River in central New Mexico. The Chinle and Dockum are thought by some to be two separate units, deposited in separate depositional basins.
for this formation. The Dockum was named by Cummins for a good exposure in vicinity of town of Dockum in Dickens County, TX in 1890. Lucas included the Dockum Group within the Chinle when he raised the Chinle to group status. Most researchers still maintain the Chinle is a formation and separate from the Dockum Group. It should be pointed out that the Dockum was named before the Chinle, and if Lucus is correct, his "Chinle Group" should be named the Dockum Group due to stratigraphic nomenclature rules.
s. The basal unit is the Santa Rosa Sandstone, a braided stream channel-related facies
. The Santa Rosa is overlain by the Tecovas Formation, which is dominated by overbank (distal floodplain
) deposits with lenses of channel-deposits. Minor lucustrine deposits are also occurring. The Trujillo Sandstone, channel-deposits, and Cooper Canyon Formation, overbank deposits with minor channel and lucustrine deposits, are separated from the Santa Rosa-Tecovas by an unconformity. In eastern New Mexico, the Redonda Formation overlies the Cooper Canyon Formation. The Redonda has gradational eastward transition into the upper Cooper Canyon Formation.
The Santa Rosa-Tecovas sequence has sediments made up of clasts derived from the north, northeast, and east of the Dockum, very similar to clasts found in the lower Chinle. However, the Trujillo-Cooper Canyon sequence’s sediments are derived from the Ouachita orogenic
belts of the Marathon Uplift
.
s. Simplified Chinle stratigraphy
based on Litwin.
Late Triassic
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...
(approximately late Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...
through Rhaetian
Rhaetian
The Rhaetian is in geochronology the latest age of the Triassic period or in chronostratigraphy the uppermost stage of the Triassic system. It lasted from 203.6 ± 1.5 to 199.6 ± 0.6 million years ago...
, or 223-200 Ma) geologic group found primarily on the Llano Estacado
Llano Estacado
Llano Estacado , commonly known as the Staked Plains, is a region in the Southwestern United States that encompasses parts of eastern New Mexico and northwestern Texas, including the South Plains and parts of the Texas Panhandle...
of western Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
and eastern New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
with minor exposures in southwestern Kansas
Kansas
Kansas is a US state located in the Midwestern United States. It is named after the Kansas River which flows through it, which in turn was named after the Kansa Native American tribe, which inhabited the area. The tribe's name is often said to mean "people of the wind" or "people of the south...
, eastern Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
, and Oklahoma
Oklahoma
Oklahoma is a state located in the South Central region of the United States of America. With an estimated 3,751,351 residents as of the 2010 census and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...
panhandle. The Dockum reaches a maximum thickness of slightly over 650 m but is usually much thinner. The Dockum rests on an unconformity
Unconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
over the Anisian
Anisian
In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from 245 million years ago until 237 million years ago, approximately...
(242-234 Ma) aged Anton Chico Formation. The Dockum and Chinle Formation
Chinle Formation
The Chinle is a geologic formation that is spread across the U.S. states of northern Arizona, Nevada, Utah, western New Mexico, and western Colorado. The Chinle is controversially considered to be synonymous to the Dockum Group in eastern Colorado, eastern New Mexico, southwestern Kansas, the...
were deposited roughly at the same time and share many of the same vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s and plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
fossils. They appear to have very similar paleoenviroment. The two units are approximately separated by the Rio Grande River in central New Mexico. The Chinle and Dockum are thought by some to be two separate units, deposited in separate depositional basins.
History of investigation
There is no designated type localityType locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
for this formation. The Dockum was named by Cummins for a good exposure in vicinity of town of Dockum in Dickens County, TX in 1890. Lucas included the Dockum Group within the Chinle when he raised the Chinle to group status. Most researchers still maintain the Chinle is a formation and separate from the Dockum Group. It should be pointed out that the Dockum was named before the Chinle, and if Lucus is correct, his "Chinle Group" should be named the Dockum Group due to stratigraphic nomenclature rules.
Stratigraphy
Lehman (1994) advocated a simplified stratigraphy of up to five geologic formationGeologic formation
A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...
s. The basal unit is the Santa Rosa Sandstone, a braided stream channel-related facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
. The Santa Rosa is overlain by the Tecovas Formation, which is dominated by overbank (distal floodplain
Floodplain
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...
) deposits with lenses of channel-deposits. Minor lucustrine deposits are also occurring. The Trujillo Sandstone, channel-deposits, and Cooper Canyon Formation, overbank deposits with minor channel and lucustrine deposits, are separated from the Santa Rosa-Tecovas by an unconformity. In eastern New Mexico, the Redonda Formation overlies the Cooper Canyon Formation. The Redonda has gradational eastward transition into the upper Cooper Canyon Formation.
The Santa Rosa-Tecovas sequence has sediments made up of clasts derived from the north, northeast, and east of the Dockum, very similar to clasts found in the lower Chinle. However, the Trujillo-Cooper Canyon sequence’s sediments are derived from the Ouachita orogenic
Ouachita orogeny
The Ouachita orogeny was a mountain building event that resulted in the folding and faulting of strata currently exposed in the Ouachita Mountains...
belts of the Marathon Uplift
Marathon Uplift
The Marathon Uplift is a Tertiary-age domal uplift, approximately in diameter, in southwest Texas. The Marathon Basin was created by erosion of Cretaceous and younger strata from the crest of the uplift...
.
Dockum/Chinle Correlation
Correlations and ages based on Luscas' Land Vertebrate Faunachrons. The faunachrons are based on first and last appearances of phytosaurPhytosaur
Phytosaurs are an extinct group of large semi-aquatic Late Triassic archosaurs. Phytosaurs belong to the family Phytosauridae and the order Phytosauria. They were long-snouted and heavily armoured, bearing a remarkable resemblance to modern crocodiles in size, appearance, and lifestyle, an example...
s. Simplified Chinle stratigraphy
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
based on Litwin.
| Sub-stage | Faunachron | Chinle | Dockum |
|---|---|---|---|
| Late Norian Norian The Norian is a division of the Triassic geological period. It has the rank of an age or stage . The Norian lasted from 216.5 ± 2.0 to 203.6 ± 1.5 million years ago. It was preceded by the Carnian and succeeded by the Rhaetian.-Stratigraphic definitions:The Norian was named after the Noric Alps in... -Rhaetian Rhaetian The Rhaetian is in geochronology the latest age of the Triassic period or in chronostratigraphy the uppermost stage of the Triassic system. It lasted from 203.6 ± 1.5 to 199.6 ± 0.6 million years ago... |
Apachean | Rock Point/Church Rock | Redonda |
| Early to Middle Norian | Revueltian | Owl Rock Upper Petrified Forest |
Cooper Canyon |
| upper Late Carnian Carnian The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian... |
Adamanian | lower Petrified Forest Moss Back Monitor Butte |
Trujillo Tecovas |
| lower Late Carnian | Otischalkian | Shinarump Temple Mountain |
Santa Rosa |