ABIQUIU - NEW MEXICO, USA
APR 7TH 2020
Ghost Ranch is located within the Chama Basin in the eastern edge of the Colorado Plateau, the basin is cut into by the Rio Chama River along with its tributaries. The Chama Basin is located between the western margin of the Great Rift zone and the eastern edge of the San Juan Basin in northern New Mexico. The southeastern section of the basin plunges gently in a northwestern direction, the southeastern margin is bordered by various northeast and north-trending faults.
Deformation of the Chama Basin began 75 million years ago during the compressional Laramide Orogeny, The orogeny was a major tectonic episode which created profound degrees of structural disruption mainly through the Rocky Mountain fold and thrust Belt up towards Canada and down towards Colorado. This event was triggered by the subduction of the Farallon Plate beneath the North American continent, the plate had a low subduction angle called Flat Slap subduction.
The Chama basin could be called a platform or Structural Terrace because of how shallow the basin formation is. The Chama Basin is a mix of predominantly red mudstone, lesser orange siltstone, conglomerate and sandstone. The basin is formed from a mix of rocks from Pennsylvanian age and to the current Holocene age, the most important age for Paleontology is the Triassic which is split into 5 different formations within the basin:
Oldest to Youngest (Triassic)
1.) Shinarump Formation
2.) Salitral Formation
3.) Poleo Formation
4.) Petrified Forest Formation
5.) Rock Point Formation
All 5 formations are put into the Chinle Group which is overlaid by the Jurassic Entrada Sandstone and its massive cliff-forming structures.
Majority of the rock layers in the Chama Basin are sedimentary in origin with the massive Entrada Sandstone overlying the basin in Ghost Ranch. Above the Entrada Sandstone is the Todilto Formation which is a mix of 3 members, the Limestone Luciano Member (5-20 feet thick), the Gypsum Tonque Arroyo Member (30-100 feet thick) and the Toldilto Breccia Member which is reserved for only Quay County Southwest Colorado. The whole Toldilto Formation is approximately 159 million years old (Late Jurassic).
There has been controversy over the deposition environment of the Todilto Formation. One side believes the formation was deposited in a Lacustrine lake environment, the other side of the debate believes that the formation originated in a moderately deep, oxygen-poor saline lake which received a substantial influx of seawater isolated from the main portion of the Jurassic Ocean.
Limestone in the Luciano Member was deposited seasonally in layers each layer represents one year with the Limestone member taking around 14,000 years to accumulate, it was a mix of Limestone, clay and dark organic material. Calcite in the Limestone Member tapers out and is replaced by gypsum, the gypsum in the above Tonque Arroyo Member is both nodular and poorly laminated. As the lake waters concentrated from evaporation, gypsum precipitated and formed the younger Gypsum Member.
Rock Point Formation
The Rock Point Formation is considered the youngest strata in the Chinle group and was deposited during the Late Triassic in the third and final epoch (237-201 million years ago). The strata in the Rock Point Formation is 70 meters thick at Ghost Ranch and is comprised of reddish-brown and greyish red beds of massive siltstone interbedded with fine-grained sandstone along with several layers of conglomerate but is mostly considered siltstone.
The fine grain quartzite sandstone contains laminated ripple patterns and was formed in a changing deposition environment which forms different sedimentary rocks, this puts the strata as cyclic as the first sandstone layer repeats more than once throughout the formation. The Rock Point Formation contains the Ceolphysis Quarry, the quarry is located in abandoned stream deposits that were once a part of a siltstone overbank sequence.
Strata in the Rock Point Formation forms ribbed cliffs due to differential weathering as some layers erode faster than others. The Rock Point Formation is usually 50-100 meters thick but does very further north in Southwestern Colorado as the formation is measured to 300 meters thick there.
The Entrada Sandstone has a sharp contact with the Rock Point Formation below. The Sandstone was deposited during the Middle Jurassic period (2nd Epoch - 176-161 Million Years old). The Entrada Sandstone is split into 3 members, the oldest being the Iyanbito Member the middle is the Medial Member and the youngest is the Upper Sandy Member.
The Entrada Sandstone contains very few fossils as do all Jurassic strata in the state of New Mexico, the reason this formation has less is that the sandstone was formed by a vast eolian dune system. Cross-beds several feet high are visible within all the sandstone members, this represents sand dunes.
Predominant vertical jointing is visible all through the Entrada Sandstone in Ghost Ranch, the jointing patterns create rectangular weathered rocks which then fall to the base of the mesa. The jointing patterns are visible in the Zuni Mountains and to the northwest in Arches National Park, the majority of the Arches and Windows form in the Entrada Sandstone in Eastern Utah although not in Ghost Ranch.
There are 4 quarries located in Ghost Ranch, the most famous is the Coelophysis (Whitaker) Quarry. Charcoal visible in several of the quarries represents a past forested area where wildfires were common over the warmer seasons, aquatic fossils have been found in all 4 quarries which means the area was most likely a swamp or river environment.
The 3 lesser-known quarries are located within the Painted Desert Member of the Petrified Forest Formation in the Chinle Group, the formation is comprised of a lower sandstone member and an upper mudstone member mixed with Bentonite (altered volcanic ash) and is reddish-brown in colour.
The Painted Desert Member is mudstone-dominated and is around 176 meters thick on average in the Chama Basin, while the lower Mesa Montosa Member is comprised mostly of Sandstone and is only 22 meters thick.
1.) Coelophysis (Whitaker) Quarry
2.) Syder Quarry
3.) Canjillon Quarry
4.) Hayden Quarry
The Syder Quarry is located in strata from the Painted Desert Member of the Petrified Forest Formation, the location was found by Mark Syder in 1998 as there was abundant charcoal on the surface. The quarry holds a Late Triassic bonebed which contains marine and land-based animals such as Phytosaurs, Aetosaurs, Rauisuchain and Eucoelophysis. Skeletons within the quarry show no signs of scavenging or weathering with minimal transport in flowing water, this suggests that burial occurred shortly after death.
The Canjillon Quarry is also located within the Painted Desert Member of the Petrified Forest Formation and was excavated by the University of California Museum Of Natural History in 1928, 1930 and 1933. The quarry lies on the same stratigraphic level as the Syder and Hayden Quarries. 11 well-preserved Phytosaur skulls were found in the location which makes them the most common fossil in the area, most were young adults but other larger adults and juveniles were also found. Aetosaurs and Metopsaurs were found there and the area lacks charcoal altogether.
Coelophysis (Whitaker) Quarry
The Coelophysis bonebed site was Discovered in 1947 by George Whitaker when he found dinosaur bones in the Upper Triassic Rock Point strata during an expedition in the current Ghost Ranch area.
A field party from the American Museum Of Natural History led by Edwin H. Colbert started an excavation of the area, the museum excavated the site in 1947, 1948, 1949, 1951 and 1953. After several years of inactivity, the quarry was reopened by the Carnegie Museum Of Natural History, led by David S. Berman during 1981 and 1982. Today the quarry remains inactive and backfilled until possible future excavations reopen it again in the future.
The Quarry held a rather thin bonebed which contained likely over a 1,000 skeletons of Coelophysis Bauri, in the end, 28 blocks were removed from a 30 cubic meter zone and sent all over the world from Ghost Ranch. The Albuquerque Natural History Museum contains one of those blocks along with the Paleontology display at Ghost Ranch. The skeletons are well preserved with about 25% of cleaned specimens being articulated or complete.
Coelophysis Bauri was a small carnivorous bipedal theropod that grew to about 6-8 feet long with some adults reaching 10 feet long. The full spectrum of growth development is visible from the individuals found within the quarry, this ranged from juveniles to fully grown adults. Predators don't usually congregate in the same area unless there is a major food source, only few fish and reptile fossils were found mixed with the Coelophysis skeletons which suggest that there was no major food source or evidence of scavenging.
The main theory of how the bonebed came into existence is that the animals were buried quickly after death, the leading hypothesis is that a flash flood killed the animals and washed them into a low lying area like a pond or a small stream system.
(Not open to the Public)
The Hayden Quarry was discovered in 2002 by Jhon Hayden a Ghost Ranch Conference Center Hiking Group Leader who was also a retired U.S Forest service Forester. The quarry is located within the stratigraphically high upper Triassic Painted Desert Member of the Petrified Forest Formation.
The bone fragments found within the quarry are mostly Phytosaurs but no large adults have been found, subadult and adult Aetosaurs are the second most common fossils in the quarry. The fragments of an unknown set of bones have been recovered with few shed teeth and a lack of cranial material which makes genetic identification rather difficult.
A Reverse Fault is located on the left side of the quarry close to the entrance which pushes the footwall down on the left side and hanging wall up on the right side. The fault remains unnamed and most likely formed from the compression of the Chama Basin during Laramide deformation in the late Cretaceous period (70-80 million years ago).
Kitchen Mesa is mainly comprised of Entrada Sandstone capped by the gypsum Toldilto Formation all of which is lying on top of the Rock Point Formation. Rocks from the thin top layer of the Toldilto Formation containing gypsum fall from the top of Kitchen Mesa, and into the exposed surface of the Rock Point Formation where the Coelophysis Quarry is located. Weathered sediment accumulates at the base of the Mesa, the material is then eroded from there into nearby streams.
Weathering & Erosion
At the base of Kitchen Mesa, there is some hoodoo like rock formations which have been formed from wind and water weathering along with Ice-wedging over the winter. The smooth surfaces represent the areas affected the most. The Rock Point Formation is eroded mostly by water which forms steep cliffs and creates crevasses which later expand into small canyons in the Ghost Ranch vicinity, the fact that the Rock Point Formation is mostly comprised of siltstone means that it erodes much differently and faster than the above layer of Entrada Sandstone.
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Spencer G. Lucas. (1995, February). Triassic stratigraphy and chronology in New Mexico. New Mexico Museum of Natural History and Science.
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