Arizona was located along the coastal edge of the continent throughout the Paleozoic era. As a result, eustatic events represent the single most important influence on Paleozoic stratigraphy throughout the state. Transgressive and regressive litholgies deposited in Arizona have been correlated to major eustatic cycles recognized world wide. These major eustatic cycles are known as Sloss sequences, named after L. L. Sloss who first recognized them. The Paleozoic rocks of Arizona record four Sloss sequences including the Cambrian to Ordovician Sauk Sequence, the Silurian to Early Devonian Tippecanoe Sequence, the Late Devonian to Mississippian Kaskaskia Sequence, and the Pennsylvanian through Permian Absaroka Sequence. Although eustatic fluctuations dominate Arizona’s Paleozoic stratigraphy, from Late Devonian time to the close of the Paleozoic, several tectonic episodes affected the state which resulted in local basin development.
Cambrian to mid-Devonian
During the first half of the Paleozoic, western North America represented a passive coastal margin. This tectonically inactive setting was characterized by carbonate platform development along the then continental shelf. Transgressive events resulted in the wide-spread distribution of carbonates across the edge of the continent in shallow epieric seas. Regressive episodes are recorded as regional unconformities broadly recognized throughout the state. Some localites, such as the Grand Canyon, record channelization during periods of extended exposure, most notably with the Ordovician-Silurian time gap, which were filled by later transgressive lithologies like the Temple Butte Formation.
Late Devonian to Permian
By Late Devonian time and continuing through the close of the Paleozoic, collisional episodes activated structural elements that provided sediment by uplift and erosion, and even guided sediment deposition by basinal subsidence. The Late Devonian to Middle Mississippian Antler orogeny and the Late Pennsylvanian and Permian Sonoman orogeny directly affected the western United States. Although no igneous and metamorphic rocks associated with these orogenies are recognized in Arizona, these collisional episodes caused crustal warping in Arizona that resulted in local uplift and subsidence. Additionally, the Ouachita orogeny, which formed in response to a collision along the southern and southeastern margin of the United States, also caused local uplift and subsidence during the close of the Paleozoic (Pennsylvanian and Permian).
Paleogeography
courtesy of Ron Blakey
- Cambrian
A transgressive sequence associated with the Sauk inundated the coastal plain of the western United States resuslting in carbonate platform development. Deposition of the Tonto Group of central and northern Arizona and genetically analagous rocks in southern Arizona (the Bolsa Quartzite, Cornado Sandstone, Abrigo Formation, and El Paso Limestone) record shoreline, lagoonal and open ocean conditions.
- Ordovician - Silurian - Early and Mid-Devonian
During this interval, Arizona was largely above sea level as recognized by a regionally extensive erosional surface (unconformity). The only Ordovician rocks in the state are located in the northwest as part of the marine Pogonip Limestone and the southeast as par of the upper El paso Limestone. No Silurian rocks have been reported from the state. The Early Devonain is also lacking, however, mid-Devonian rocks have been reported from central Arizona and are significant because they contain early plant remains and suggest estuarine to riparian deposition.
- Mid- to Late Devonian
The Mid- to Late Devonian marks a significant transition in deposition across the state of Arizona as the first of a series of orogenic episodes began to affect the state. The Antler Orogeny mainly influenced central Nevada and western Utah but is probably also responsible for flexural warping in northern Arizona. Accompanying tectonism, a general transgressive phase is recorded throughout the Devonian ending during latest Devonian time. Local basins and troughs flanked by structural highs influenced deposition which is largely characterized by near-shore marine and tidal flat deposition best represented by the Martin Formation.
- Mississippian
- Pennsylvanian
Early Mississippian time in the western United States is largely dominated by the final throws of the Antler Orogen and the emplacement of the Roberts Mountains Allochthon in central Nevada. To the east of the Antler mountain belt a deep foreland basin developed and the western margin of the United States isostatically subsided under the weight of accumulating sediments. Transgressing seas aided sediment accumulation and all of Arizona was inundated except for topographic highs along the Transcontinental Arch in the northeastern corner of the state. The Redwall Limestone, a crinoid-rich marine deposit, is arguably the most extensive deposit in Arizona geologic history as it is recognized across nearly 50% of the state excluding southeastern Arizona where it grades laterally into the Escabrosa Limestone.
Deposition during the Pennsylvanian is charcterized by local basin development as influenced by the assembly of Pangea and the uplift of the Ancestral Rocky Mountains. Two orogenic episodes influenced basin development: the Sonoman Orogeny along the west coast and the Ouachita Orogeny along the Gulf coast. Subsiding basins in Arizona record interbedded marine and coastal plain deposits as sea level fluxed in and out of the depositional centers. The basal Supai Group, basal Naco Group, and Hermosa Group define these conditions. During some regressive phases, evaporite deposits formed within the more interior basins (four-corners area). To the west, marine deposition dominated until the Late Pennsylvanian when the Sonoman and regressive conditions resulted in uplift and exposure.
- Permian
The Sonoman and the Ouachita orogenies continued to direct sedimentation in Arizona during the Permian. Early Permian deposition was dominated by coastal plain and fluvial deposits along with extensive sand dune deposits most porperly defined as "ergs" (arabic for "sand dune sea"). The sands appear to have been sourced from the Oachita mountain belt to the east and southeast and were blown northwesterly into a shallow western ocean much like the north coast of Africa today.
Southeastern Arizona witnessed the development of the Pedregosa Basin which was initated during the Late Pennsylvanian. Shallow to deep marine deposits charcterize the basin during this time and include limestones, dolostones, and black shales.
The close of the Permian, and the Paleozoic, in Arizona is characterized by state-wide marine deposition associated with transgressing seas. To the north, the Kaibab Limestone records transgressing seas from the western Grand Canyon to eastern portions of the state (near Holbrook), a distance of over 200 miles. In the south, the Rain Valley Formation is largely Kaibab equivalent and represents marine deposition as well as the youngest Permian/Paleozoic rocks in the state.
The end of the Permian is represented by a regional unconformity associated with regression, presumably the result of glaciation in southern Pangea.
Perhaps the best place in the world to study Paleozoic stratigraphy is within the Grand Canyon of Arizona. The oldest rocks in the canyon may date back as far as 2.0 Ga (Early Proterozoic), while the youngest rocks are as little as a few hundred thousand years old. Although the Grand Canyon contains the most continuous record of Earth history known, the canyon does not contain a time-continuous rock record. In fact, more time is missing from the stratigraphy than is recorded! Time gaps (unconformities) recognized in Grand Canyon stratigraphy are the result of exposure due to sea level fall, or uplift, and erosion. Note that the best represented interval of time is the Paleozoic era. Even so, this section contains numerous unconformities, some of which are only a few million years in duration while others span 50 million years or more (such as the lack of Ordovician, Silurian, Early and Middle Devonian rocks in the canyon).
As mentioned above, the lower Paleozoic rocks of Arizona reflect changes in sea level. The Cambrian Tonto Group is perhaps the best example in the Grand Canyon of a transgressive sequence. The Tonto Group is represented by the Tapeats Sandstone (a clean quartzitic sandstone with cross-bedding and bioturbation), the Bright Angel Shale (a laminated, black siltstone and shale with abundant animal fossils and bioturbation) and the Muav Limestone (a calcarenite with abundant marine fossils), and was deposited as part of the Sauk Sequence. The regressive portion of the Sauk Sequence is recognized by an unconformity which caps the Tonto Group and indicates exposure and erosion due to a sea level fall. Unconformities are often the only way to recognize regressions in the rock record because, as sea level falls, exposure and erosion removes some or all of the sediments deposited during the sea level fall. The following period of exposure can may even remove of part or all of the transgressive stratigraphy as well.
Mississipian through Permian stratigraphy of the Grand Canyon.
Photo: Ron Blakey
The remaining formations in the Grand Canyon, which span Late Devonian time to Permian time, do record transgressive and regressive events in response to eustacy. However, tectonic influences were also present thus making it difficult to recognize changes in lithology due to sea level rise and fall. In some cases, transgressive and regressive cycles can only be recognized by fossil content and more subtle features within the rocks, such as the presence of marine cements.
The Grand Canyon
photos courtesy of Ron Blakey
Paleozoic fossil assemblages, in general, are dominated by brachiopods, archaic corals (both colonial and solitary), trilobites (especially in the lower half of the Paleozoic), bryozoans, and crinoids. Several of these groups suffered extreme losses at the Permian-Triassic extinction event, some, such as the trilobites, went completely extinct. With the start of the Mesozoic, other organisms came to dominate fossil assemblages.
The Paleozoic paleontology of Arizona reflects the general trends described above. The lower Paleozoic rocks of Arizona are the result of a series of transgressive and regressive events and, as a result, the fossil assemblages of this time are largely marine. This fossil assemblage is characterized mainly by various brachiopod species, crinoids, and coral and is diagnostic not only of Arizona, but of North America in general and thus allows for extensive biostratigraphic correlation.
By the arrival of the latter half of the Paleozoic, deposition in Arizona was largely controlled by tectonic events and is reflected in local basin development. These basins recorded marine invasions as they subsided but also retained a terrestrial record as the oceans retreated. Terrestrial fossils are, therefore, rather common occurrences in Arizona and include some plant fossils and invertebrate and vertebrate tracks and trails.
The Mississippian Surprise Canyon Formation of the Grand Canyon is a lenticular deposit with brecciated basal beds and sandstones that contain Lepidodendron (fossil "tree" club moss which formed extensive coal deposits in eastern North America during the Pennsylvanian) and Calamites (an ancient relative of Equisetum, the modern horsetail or scouring rush) which has been interpreted as fluvial in origin. These basal beds are overlain by fossiliferous micrites and fossiliferous calcarenites and chert that include solitary and colonial corals, starfish, brachiopods, and clams all of which indicate a marine invasion drowned the ancient river valley.
The Coconino Sandstone of the southern Colorado Plateau, records abundant tracks and trails which have been assigned to scorpions, insects, and early reptiles, as they troddened interdune troughs of an extensive coastal dune complex.