The Mesozoic history of Arizona was largely dominated by orogenesis. Sonoman and Ouachita orogenesis, which began at the close of the Paleozoic, continued into the Early Triassic. Crustal folding and faulting which defined regions of uplift and subsidence initiated by these tectonic episodes continued to dominate sedimentation patterns throughout the state. Although these orogenic events affected sedimentation in Arizona, the actual orogenic front (magmatic arc and adjacent highlands) associated with each orogeny was rather distantly removed from the state (Utah-Nevada-southeastern California for the Sonoman; southern and southeastern United States for the Ouachita).
The Late Triassic and Jurassic of Arizona is characterized by the development of a volcanic arc as revealed in Jurassic volcanic and plutonic rocks in southern Arizona. Termed the Nevadan Orogeny, Arizona recorded the effects of a local orogenic front for the first time since the Mazatzal Orogeny of the Middle Proterozoic. The lack of sedimentary rocks (unconformity) in central Arizona of Triassic and Jurassic age help to define the position of highlands, called the Mogollon Highlands, which were just northward of the Nevadan magmatic arc.
The Sonoman and Nevadan orogenies were each associated with subduction. The collision, however, recorded a large strike-slip component and, as a result, the magmatic arc was largely contained within a series of northwest-southeast trending pull-apart basins. Because the Mogollon Highlands were, in part, associated with extension, they were likely not an impressive mountain system during this time.
The Mogollon Highlands were drained by the erosive forces of streams and rivers that dumped sediment onto lowlands in northern Arizona, which are now part of the southern Colorado Plateau. A thick set of terrestrial strata is recorded in the shales, siltstones, sandstones, and conglomerates in formations such as the Moenkopi, Chinle, Moenave, and Kayenta. Interbedded volcanic ash and volcaniclastic sediments derived from the south also occur within these strata.
The Cretaceous of Arizona represents a period of widespread crustal disturbance. During the Jurassic, the Gulf of Mexico began to open and by the Early Cretaceous, an arm of the developing ocean, called the Chihuahua Trough, extended into southeastern Arizona. The trough is thought to be an allocogen, the failed arm of a rift, and contains a mixture of conglomerate, sandstone, shale, and limestone, all part of the Bisbee Group, which represents the marine embayment.
The Sevier Orogeny largely affected western North America as a series of thrust belts which constructed the western half of the Rocky Mountains. It also formed a seaway, called the Cretaceous Western Interior Seaway, that stretched from the then-opening Gulf of Mexico northward to the Arctic Ocean. The seaway formed in response the Sevier orogenic belt as it advanced to the east and was eroded. Sediment from the belt accumulated in a developing trough called the Sevier foreland basin. Foreland basins form in to load as the weight of an advancing mountain front and the accumulation of eroding sediments adjacent to the front depresses the crust in a subsiding trough that parallels the orogenic belt. In Arizona, the foreland basin accumulated coastal and marine deposits along the western shoreline of the seaway, due in part to loading, but also in response to eustacy.
The close of the Mesozoic in western North America is characterized by the Laramide Orogeny which continued into the Early Cenozoic. The Laramide is primarily responsible for the construction of the eastern half of the Rocky Mountains during a phase of orogensis that began about 80 Ma and ended around 50 MA. Unlike the Sevier before it, the Laramide had a dramatic effect on the geology of Arizona. Volcanic and plutonic rocks of Larmide age are recognized in southern and southwestern Arizona and are responsible for the emplacement of ore bodies in those areas. Volcaniclastic sediments are locally abundant and reflect erosion of the magmatic arc. Thrusting and folding associated with Laramide deformation is recognized throughout the central part of the state in response to intense compression that was directed toward the northeast. The Colorado Plateau was also affected by the Laramide, as reverse faults initiated at depth below the plateau propagated upward. These deep faults, however, only fractured the Proterozoic basement rocks and caused monoclinal folds to develop in the overlying Paleozoic and Mesozoic rock layers as they draped over the faults. The Colorado Plateau contains many monoclines, such as the East Kaibab and the Organ Rock monoclines, and is home to more of these features than anywhere else in the world.
Sedimentary Responses to Tectonic Uplift
Much of the Triassic and Jurassic
sediment on the Colorado Plateau is characterized by fine-grained clastic
rocks and represents shallow, broad, meandering stream systems which were
draining the Mogollon Highlands to the south. An exception to this
is the Triassic Shinarump Conglomerate, the basal member of the Chinle
Formation. It contains deep, narrow channels with abundant pebbles
and cobbles. Thus, the Shinarump represents some change in depositional
dynamics of the Triassic system in Arizona. In order to consider
this change, we must first recognize the processes that control fluvial
deposition.
Streams and rivers erode landscapes.
They flow downstream from their headwaters to some receiving body of water
such as a lake or ocean. The course of a river decreases in elevation
from its headwaters to the point were it enters a body of water and forms
a delta. This change in elevation over the course of the river is
referred to as the stream gradient . Streams are always trying to
reduce their gradient by eroding the landscape they traverse. As
a landscape is reduced in elevation by erosion, streams lose erosive power
because the stream gradient is no longer steep enough to support fast-flowing
currents. As a result, stream channels broaden and begin to meander.
Deposition in such a setting is of fine-grained sediment because of lower
current energy. In contrast, streams with a steep gradient have stronger
current energy and can carry coarse-grained sediment in rather straight,
narrow, and deep channels.
With the above perspective in
mind, lets return to the clastic rocks of the southern Colorado Plateau.
The basal Mesozoic Formation is the Early Triassic Moenkopi Formation.
It consists of sandy siltstones, siltstone, and silty shale interpreted
to represent alluvial plain deposits associated with a meandering river
system. The Moenkopi is capped by the Chinle Formation which is divided
into several members. The basal member is the Shinarump Conglomerate
and is dominated by pebble to cobble-sized clasts and coarse sand within
narrow channels. The overlying members of the Chinle, including the
Petrified Forest and the Owl Rock Members, are dominated by shale, silty
shale, and fine-grained sandstone along with volcanic ash in shallow, broad,
and meandering channels. This sequence of terrestrial deposition
is interpreted to represent a change in stream gradient. Initially,
low gradient streams deposited the Moenkopi Formation. A change to
a higher gradient is recorded in the Shinarump channels while a return
to a lower gradient deposited the remaining members of the Chinle Formation.
Tectonically, the Shinarump is thought to represent a period of increased
elevation within the Mogollon Highlands, giving additional erosive power
to northward-flowing stream systems as their gradients were increased.
The remaining sediments within the Chinle are fine-grained, suggesting
that the uplift of the highlands was short in duration. Thus, sedimentation
cycles, such as the Triassic rocks of the southern Colorado Plateau, can
record tectonic events associated with sediment sources.
Mesozoic Rocks Exposed at Black Mesa
The Cretaceous stratigraphy
exposed at Black Mesa in northeastern Arizona records a transition from
continental, to coastal, and then marine deposition in a series of clastic
rock strata. The marine invasion responsible for this change, unlike
those of the Paleozoic, advanced from east to west as Sevier and Laramide
orogenesis developed a broad foreland basin that flanked the growing Rocky
Mountains. As orogenisis proceeded, clastic material was shed from
the mountain front eastward into the subsiding foreland basin which occupied
the western plains.
During episodes of active mountain building, the
proximal portions of a foreland basin, (those adjacent to the mountain
belt) trap coarse clastic sediment as that portion of basin subsided rapidly
under the recently advanced weight. At the same time, mid- to distal
portions of the basin received fine-grained clastic sediment. Coarse
sediment can reach the distal portion of foreland basin between episodes
of active thrusting. During this time of inactivity, the orogenic
belt erodes filling the proximal foreland basin. Once filled with
sediment, coarse clastics spread into the mid- and distal portions of the
basin.
The stratigraphy at Black Mesa
is largely fine-grained because it represents the mid- distal potions of
the Sevier-Laramide foreland basin, far from the advancing orogenic front
and because mountain-building and foreland basin development was so rapid.
Some of the coarse-grained rocks at Black Mesa, however, indicate episodes
between mountain building.
Mesozoic Paleogeography
courtesy of Ron Blakey
- Early Triassic
The Mesozoic began during a time of continental assembly and glaciation in southern Pangea (Gondwana). As a result, a world-wide sea level low left most of the western United Staes exposed. Arizona is one of the few places where terrestrial sediments of Mesozoic age are so well preserved as the northern part of the state receiving sediments shed from topographic highs to the south and southeast. At the start of the Triassic, sedimnentation was fine-grained, represented by silts and fine sands which formed the Moenkopi Formation.
- Late Triassic
Orogenic processes in southern Arizona uplifted mountains alogn a transtensional margin. Withn new-found relief, river systems which drained the mountain system were invigorated and coarse clastics were shed great distances to the north. In northern Arizona, the Shinarump Member of the Chinle Formation contains meta-clasts of pebble to cobble-sized quartzites and gneiss derived from the south along with well defined and cross-bedded channels and petrified wood. The remainder of the Chinle Formations is typified by very fined-grained sediments, silts, clays, and volcanic ash, as the magmatic arc "relaxed" and subsided, thus reducing stream vigor and topography in the southern part of the state.
- Middle Jurassic
Transtensional orogensis along the southern edge of Arizona continued during the Jurassic as part of the Nevadan Orogeny. The Mogollon Highlands, an uplifted terrane flanking the orogen, shed material northward. Sand dunes migrated across much of the western United States, including northern Arizona, in huge ergs. The Navajo Sandstone is an example of deposition during this time and records massive cross-bedding, interdune lime ponds and dinosaur tracks. Pangea began to breakup during time as well.
- Early Cretaceous
The Sevier Orogeny constructed an impressive magmatic arc along the west coast of the United States, the remains of which are now exposed in the Sierra Nevada Batholith. Inland of the arc, arc-derived sediments accumulated in a deep and isostatically subsiding basin; the Sevier Foreland Basin. Under the weight of accumulating sediments, the crust of central North America, between what is now the eastern Rocky Mountains and the Appalachais, was depressed downward toward sea level. A contemporaneous sea level rise resulted in the inundation of this corridor which linked the Arctic Ocean with the Gulf of Mexico with the Creataceous Interior Seaway. Deposition associated with this seaway and the Sevier Foreland Basin is observed in the eastern and northeastern portions of the state near St. Johns and at Black Mesa, respectively.
Continued fragmentation of Pangea resulted in the opening of the Chihuahua Trough and the Pedregosa Basin in extreme southeastern Arizona. These depositional centers contain strata which record the opening Gulf of Mexico. The Mogollon Highlands contiunued to remain a positive feature in the central part of the state.
Mesozoic Fossils of Arizona
Arizona contains a diverse assemblage
of both marine and terrestrial organisms from the Mesozoic. During
the Triassic and Jurassic, most of Arizona represented terrestrial environments
that where inhabited by a diverse array of plants, a variety of reptiles,
including dinosaurs, and amphibians. These environments are best
preserved on the Colorado Plateau in the northeastern corner of the state.
The flora during this time was dominated by conifers, ferns, and cycads
and is best represented by Woolgaria arizonicum, a cycad from the
basal Chinle Formation, and Araucarioxylon, a spruce from the Petrified
Forest Member of the Chinle Formation. Often, plant remains in the
Chinle occur as isolated lenses of carbon rich shales suggesting deposition
in stagnant freshwater conditions. Plant material has also been recovered
from the Navajo Sandstone.
Vertebrate fossils in Triassic
and Jurassic rocks of the plateau dominate the fauna although some freshwater
invertebrates
(mollusks) have been reported from the Moenkopi Formation. Additionally,
the Moenkopi is one of many formations world-wide which contains
Cheirotherium
tracks, an odd reptile which had its outer digit, equivalent to our "little
finger", protruding laterally at nearly a right angle to the axis of the
vertebral column. Phytosaurs, a crocodile-like reptile, and metopasaurs
which are large amphibians, are the most common vertebrates known from
the Chinle Formation. Dinosaur remains and tracks occur in the Moenave
Formation, Kayenta Formation, and the Navajo Sandstone. Dilophosaurus,
an early theropod (carnivorous) dinosaur and proposed Arizona state fossil,
has been recovered from both the Moenave and Kayenta Formations, the former
of which has extensive trackways, termed Dilophasaurapus, assigned
to the dinosaur. Many different dinosaur tracks have been identified
in these formations including Tyranosaurids from the Navajo Sandstone.
Fish remains have also been identified from the Chinle and the Moenave
Formations and round out the fauna. The Chinle fish are lobe-finned
fish and are related to the modern day coelacanth (Latemeria).
By the Cretaceous, the development
of the Cretaceous Western Interior Seaway and the Pedregosa Basin spread
marine environments into the northeastern and southeastern corners of the
state respectively. Coal beds occur at Black Mesa in northeastern
Arizona in advance of marine layers and contain conifer (Sequoia)
and angiosperm (Diospyros) remains. Angiosperms (flowering
plants), evolved during the late Jurassic and underwent an adaptive radiation
during the Cretaceous. Today, they are the dominant land plants.
Invertebrates representative of the Cretaceous sea include foraminiferans,
scleractinian corals, gastropods, rudist bivalves and other clams, a wide
variety of oysters, and ammonites. Dinosaur remains are also known
from the Cretaceous and include a newly discovered sauropod and proposed
state fossil Sonorasaurus from the southeastern part of the state.
Marine reptiles, such as plesiosaurs and ichthyosaurs, and sharks have
been recovered in the northeastern portion of the state and represent oceanic
predators.
Mesozoic Outcrop Photos
courtesy of Ron Blakey