In geology, a group is a lithostratigraphic unit consisting of a series of related formations that have been classified together to form a group. Formations are the fundamental unit of stratigraphy. Groups may sometimes be combined into supergroups .
10-555: The Spears Group is a group of geologic formations exposed in and around the northeast Mogollon-Datil volcanic field of southwestern New Mexico . It has a radiometric age of 33 to 39 million years, corresponding to the Eocene to Oligocene epochs . The group is composed primarily of volcaniclastic beds, deposited in alluvial fans or braided streams , with minor basaltic andesite to dacite lava flows. It contains some lacustrine delta deposits at its base. The total thickness
20-558: Is as much as 3,000 feet (910 m). The group grades below into the Baca Formation and interfingers with lava flows and ash flow sheets of the Mogollon Group and Datil Group . K-Ar and fission track dating suggest an age of 33 to 39 million years. The Dog Springs Formation shows striking sedimentary structure indicating that the debris flow beds making up most of the formation slumped over distance scales of miles on
30-571: The Glenwoody Formation , other strata (particularly in the lower part of the group) remain undivided into formations. Some well known groups of northwestern Europe have in the past also been used as units for chronostratigraphy and geochronology . These are the Rotliegend and Zechstein (both of Permian age); Buntsandstein , Muschelkalk , and Keuper ( Triassic in age); Lias , Dogger , and Malm ( Jurassic in age) groups. Because of
40-820: The Wingate Sandstone , the Moenave Formation , the Kayenta Formation , and the Navajo Sandstone . Each of the formations can be distinguished from its neighbor by its lithology , but all were deposited in the same vast erg . Not all these formations are present in all areas where the Glen Canyon Group is present. Another example of a group is the Vadito Group of northern New Mexico . Although many of its strata have been divided into formations, such as
50-628: The confusion this causes, the official geologic timescale of the ICS does not contain any of these names. As with other lithostratigraphic ranks, a group must not be defined by fossil taxonomy. Malm (geology) The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata . In European lithostratigraphy ,
60-544: The name " Malm " indicates rocks of Late Jurassic age. In the past, Malm was also used to indicate the unit of geological time, but this usage is now discouraged to make a clear distinction between lithostratigraphic and geochronologic/chronostratigraphic units. The Late Jurassic is divided into three ages, which correspond with the three (faunal) stages of Upper Jurassic rock: During the Late Jurassic Epoch, Pangaea broke up into two supercontinents , Laurasia to
70-520: The north, and Gondwana to the south. The result of this break-up was the spawning of the Atlantic Ocean . However, at this time, the Atlantic Ocean was relatively narrow. This epoch is well known for many famous types of dinosaurs , such as the sauropods , the theropods , the thyreophorans , and the ornithopods . Other animals, such as some crocodylomorphs and the first birds , appeared in
80-460: The stratigraphy of large regions. Geologists exploring a new area have sometimes defined groups when they believe the strata within the groups can be divided into formations during subsequent investigations of the area. It is possible for only some of the strata making up a group to be divided into formations. An example of a group is the Glen Canyon Group , which includes (in ascending order)
90-590: The underlying Baca Formation beds. In some locations, clastic dikes derived from the Baca Formation penetrate the overlying Dog Springs Formation. The group is divided into numerous formations. In descending stratigraphic order, these are: In addition, a number of informal units have been described within the group. The name was first used by W.H. Tonking in 1957 for the lowest beds of the Datil Volcanics (as then designated). G.R. Osburn and C.E. Chapin raised
100-722: The unit to formation rank within the Datil Group and divided into members separated by ash flow sheets. In 1994, Steven M. Cather and coinvestigators raised the Spears Formation to group rank and its members to formation rank, redefining the group as all volcaniclastic apron sediments of the Mogollon-Datil volcanic field. They also described several informal formations within the group. Group (stratigraphy) Groups are useful for showing relationships between formations, and they are also useful for small-scale mapping or for studying
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