Spears Group | |
---|---|
Stratigraphic range: Eocene to Oligocene | |
Type | Group |
Sub-units | See text |
Underlies | Fence Lake Formation Hayner Ranch Formation [1] |
Overlies | Baca Formation |
Thickness | 3,000 ft (910 m) |
Lithology | |
Primary | Volcaniclastics |
Other | Andesite |
Location | |
Coordinates | 34°16′52″N107°17′46″W / 34.281°N 107.296°W |
Region | New Mexico |
Country | United States |
Type section | |
Named by | Tonking |
Year defined | 1957 |
The Spears Group is a group of geologic formations exposed in and around the northeast Mogollon-Datil volcanic field of southwestern New Mexico. [2] [3] 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. [3] It contains some lacustrine delta deposits at its base. [4] The total thickness is as much as 3,000 feet (910 m). [3] The group grades below into the Baca Formation [4] and interfingers with lava flows and ash flow sheets of the Mogollon Group and Datil Group. [3]
K-Ar and fission track dating suggest an age of 33 to 39 million years. [3]
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 the underlying Baca Formation beds. In some locations, clastic dikes derived from the Baca Formation penetrate the overlying Dog Springs Formation. [5]
The group is divided into numerous formations. In descending stratigraphic order, these are: [6]
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). [2] G.R. Osburn and C.E. Chapin raised the unit to formation rank within the Datil Group and divided into members separated by ash flow sheets. [3] 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. [6]
The Thirtynine Mile volcanic area, part of the larger Central Colorado volcanic field, is an extinct volcanic area located in Park and Teller counties, Colorado, northwest of Cripple Creek and southeast of South Park. The area was the site of significant volcanism in the Paleogene Period about 35 million years ago. Ashfall and lahars (mudflows) from the volcanoes created the conditions for fossilization at what is now Florissant Fossil Beds National Monument.
The Kneeling Nun Tuff is a geologic formation exposed in southwest New Mexico. It has a radiometric age of 35.3 million years, corresponding to the latest Eocene epoch.
The Mogollon-Datil volcanic field is a large silicic volcanic field in western New Mexico. It is a part of an extensive Eocene to Oligocene volcanic event which includes the San Juan volcanic field in southwestern Colorado, the Trans-Pecos volcanic field in west Texas and north central Mexico, the Boot Heel volcanic field in the bootheel of southwestern New Mexico and adjacent areas of Arizona and Mexico; and the vast volcanic field of the Sierra Madre Occidental of western Mexico. The Mogollon-Datil volcanic field was formed in "four discrete pulses representing synchronized activity of two separate cauldron complexes".
The Lake Valley Limestone is a geologic formation widely exposed in southwestern New Mexico. It preserves fossils dating back to the lower to middle Mississippian.
The Popotosa Formation is a geologic formation in New Mexico. It preserves fossils dating back to the Neogene period. These include the Socorro flora, notable for its fine preservation of plant reproductive structures.
The Sierra Ladrones Formation is a geologic formation exposed near the Rio Grande Valley in New Mexico. It preserves fossils of Pliocene to Pleistocene age.
The Baca Formation is a geologic formation in southern New Mexico and Arizona. It preserves fossils dating back to the Eocene period.
The Cub Mountain Formation is a geologic formation in southern New Mexico. It preserves fossils dating back to the Eocene epoch. The formation also records the progressive unroofing of nearby mountainous uplifts during the Laramide orogeny.
The Hart Mine Formation is a geologic formation controversially defined from exposures in south-central New Mexico. It preserves fossils dating back to the Eocene epoch.
The Palm Park Formation is a geologic formation in southern New Mexico. It preserves fossils dating back to the Eocene epoch.
The Rubio Peak Formation is a geologic formation in southwestern New Mexico. It is thought to have been deposited in the Eocene Epoch.
The geology of New Mexico includes bedrock exposures of four physiographic provinces, with ages ranging from almost 1800 million years (Ma) to nearly the present day. Here the Great Plains, southern Rocky Mountains, Colorado Plateau, and Basin and Range Provinces meet, giving the state great geologic diversity.
The Hayner Ranch Formation is a geologic formation found near the San Diego Mountains of New Mexico. It is estimated to have been deposited during the Miocene epoch.
The Love Ranch Formation is a geologic formation in southern New Mexico. It was likely deposited during the late Paleocene and early Eocene epochs.
The Bell Top Formation is a geologic formation in southern New Mexico. Radiometric dating of surrounding tuffs indicate that it was deposited in the Oligocene epoch.
The Oligocene Chuska Sandstone is a geologic formation that crops out in the Chuska Mountains of northeastern Arizona and northwestern New Mexico. The formation is a remnant of a great sand sea, or erg, that once covered an area of 140,000 square kilometres (54,000 sq mi) reaching from the present locations of the Chuska Mountains to near Albuquerque and to the southwest. This erg deposited a succession of sandstone beds exceeded in thickness only by the Navajo Sandstone on the Colorado Plateau.
The Sanders Canyon Formation is a geologic formation exposed north and west of the Sierra Blanca of southern New Mexico. It preserves fossils dating back to the Eocene epoch.
The Bearwallow Mountain Andesite or Bearwallow Mountain Formation is a geologic formation exposed in and around the Mogollon Mountains of southwest New Mexico. It has a radiometric age of 27 to 23 million years, corresponding to the late Oligocene to early Miocene epochs.
The Fence Lake Formation is a geological formation in western New Mexico whose strata were deposited in the Miocene.