12.1: Geography and Tectonics of the Early Paleozoic

Overview

At the start of the Paleozoic, in the Cambrian Period, the continents were near or below the equator, making their configuration essentially opposite of the world today. Laurentia (proto North America) had recently separated from the supercontinent Gondwanaland, and was situated along the equator and mostly above sea level. As the Paleozoic progressed, the global climate warmed, melting glaciers and sea levels rose. The lithology of sedimentary rocks deposited during this time tells the story of sea level transgressions during the early part of this Era. Near the end of the Ordovician, sea levels regressed as Gondwanaland moved across the South Pole and an ice sheet grew across the cratons. This relatively short glaciation period caused one of the greatest extinctions of marine organisms (Stanley and Luczaj, 2015).

Figure \(\PageIndex{1}\): Laurentia (ancestral North America) during the Cambrian with the state of Texas for reference. Credit: Ian Dalziel, University of Texas at Austin.

North American Continent

The eastern margin of the ancestral North American continent was active, with subduction driving mountain building. The first of three mountain-building events that constructed the Appalachian Mountains, the Taconian Orogeny, happened as Laurentia collided with several islands situated between the continent and Gondwanaland.

In contrast to the East Coast, the West Coast was a tectonically inactive passive margin during the early Paleozoic. The western edge of the North American continent was near the present-day Nevada-Utah border and formed an expansive shallow continental shelf near the paleoequator. Starting in the Devonian, the west coast transformed into an active continental margin as an island arc collision drove the Antler Orogeny. This marked the start of subduction and accretion of continental material onto the western margin of North America, which would eventually become portions of the states of California, Oregon, and Washington. The Sonoma orogeny began in the late Paleozoic when subduction caused North America to collide with another island arc. Eventually, a subduction zone was established along the entire western margin that persisted through the Mesozoic Era.

The video below summarizes the tectonic and environmental changes taking place in North America during the Paleozoic. On the left side, sea level transgressions and regressions are indicated by blue and brown, respectively. The transgressive sequences are named after the locations where their deposits were originally studied: Sauk, Tippecanoe, Kaskaskia, and Absaroka. To the right is a geologic timeline and the geographic evolution of North America.

The Wilson Cycle

Canadian geophysicist J. Tuzo Wilson developed a model that explains how an ocean basin opens and closes, beginning with a divergent boundary and ending with collisional subduction between two continents. This model was developed using observations of fossil similarities on either side of the Atlantic and structural evidence that the Atlantic Ocean opened roughly where an ancient ocean had closed. This led Wilson to propose that a “proto-Atlantic Ocean” existed between the continents during the Paleozoic Era. This ocean closed in stages, bringing together dissimilar rocks as the supercontinent Pangaea formed.

The Sauk, Tippecanoe, Kaskaskia, and Absaroka sequences were deposited during one very long period of episodic deposition, marking the opening and closing of the Iapetus Ocean as illustrated by the Wilson Cycle. This simple model can also be applied to other contexts, such as the closing of the Tethys Sea as India moved northward toward Asia. Watch this simple schematic video of the Wilson Cycle below.

Stages of the Wilson Cycle

A Wilson Cycle has nine stages, as described in the diagrams below. Stage A begins with a stable continental craton and progresses to Stage B, where continental rifting begins. This rifting progresses through stages C through E, with passive margins on both sides of the new ocean basin.

The closing of the rift basin begins with Stage F and then runs through Stage I, where the two continents come together to form a new supercontinent. In the meantime, Stages E through H record the formation of orogenic events and the accretion of terranes. The Sauk, Tippecanoe, Kaskaskia, and Absaroka sequences fit in the model from stage C to before stage H.

Figure \(\PageIndex{2}\): Stages A through E of a Wilson Cycle (Whitmeyer et al., 2007)

Figure \(\PageIndex{3}\): Stages F through I of a Wilson Cycle (Whitmeyer et al., 2007).

The Sauk and Tippecanoe Transgressive Sequences

The Sauk, Tippecanoe, Kaskaskia, and Absaroka marine transgressions deposited sediment across most of North America from the Cambrian through the Permian Periods. In the late Proterozoic, sea levels rose as the supercontinent Rodinia broke up. Seawater flowed across Laurentia, forming the Sauk Sea, and sediment was deposited atop the long-eroded surface of the craton. This resulted in the Great Unconformity, representing a gap of geologic time ranging from 500 million to 1.2 billion years. Carbonates, shale, sandstone, and volcanic ash were deposited with each pulse of sea level rise, recording rising or falling sea levels, volcanism, and mountain building.

Here, we'll examine the formations produced during the Sauk and Tippecanoe transgressions in greater detail to see what story their rocks can tell.

Figure \(\PageIndex{4}\): A stratigraphic column summarizing the lithology and related tectonic activity of the Sauk and Tippecanoe transgressions.

At the start of the Paleozoic, the Laurentian craton was mostly above sea level as reflected by thick accumulations of siliclastic sediment (terrestrial) in early Cambrian formations, like the Chil-Howee. Cross-bedding is common in these rocks, attesting that wind played an important role in transporting and depositing sediment: a consequence of a continent devoid of vegetation. The early Ordovician New Market Limestone, as indicated by its lithology, fossils, and structure, preserves a record of a shallow sea. Examine the carbonate rocks of the New Market and Lincolnshire formations in the GIGAmacro image below.

Figure \(\PageIndex{5}\): Carbonate rocks of the New Market and Lincolnshire formations.

Later in the Ordovician, deposits from the Tippecanoe transgression reached the seafloor as a type of landslide called turbidity currents. Evidence of this is the mix of terrestrial sediment, marine carbonates, and finely preserved fossils. The GIGAmacro image below shows well-preserved graptolite fossils (darker, caterpillar-like markings) in a muddy limestone. Such preservation can result from rapid burial, as would be the case with submarine landslides.

Figure \(\PageIndex{6}\): Graptolite fossils preserved in a muddy limestone.

The Tippacanoe Sequence also reflects mountain building from the Taconian Orogeny. The Silurian Massanutten Sandstone formed from the siliclastic sediment eroded from the highlands produced by the Taconian Orogeny. This formation contains sedimentary structures like ripple marks and cross-bedding, reflecting deposition in shoreline and dune environments. See the hand sample below.

Figure \(\PageIndex{7}\): Massanutten Sandstone produced by Taconian Orogeny.

For reference, Laurentia may have looked like this during the Silurian.

Figure \(\PageIndex{8}\): Late Silurian (Taconian Orogeny) Laurentia.

With time, the Taconian mountains were eroded down to a lowland. This and renewed transgression are reflected by the transition from the coarse sands of the Massanutten to the fine-grained intertidal Bloomsburg and Tonoloway Formations, then the fossil-rich limestone of the Helderberg Formation. The latter formed as a carbonate bank of limestone and dolomite in a warm, shallow sea during the early Devonian.

Figure \(\PageIndex{9}\): Bloomsburg Formation, Fort Valley, VA.

The Tippacanoe Sequence ends with a significant sea level regression and prolonged erosion. This is marked by the Wallbridge Unconformity, upon which the Kaskaskia, and Absaroka marine transgressions deposited sediment, further adding strata atop the Laurentian craton.

The interpretation of stratigraphy and unconformities, as exemplified by examination of the Sauk and Tippecanoe sequences, can provide a model for interpreting the changes we see in facies. These changes, resulting from the complex interplay among tectonics, sedimentation, and relative sea level, constitute the story of the strata. Consequently, we know the geologic history of a basin, like the Iapetus Sea, and the paleography of a continent, like Laurentia. Through interpreting stratigraphy, rocks can tell their story!

Key Terms

• Absaroka Sequence - A group of North American sedimentary formations formed during marine transgressions in the Mississippian, Permian, Triassic, and Jurassic periods
• facies - Characteristics of a rock unit that reflect the origin of the rock and distinguish it from adjacent rock units. These characteristics include mineralogy, sedimentary structures, fossil content, and texture. These provide clues about the depositional environment in which a rock was formed
• Kaskaskia Sequence - A group of North American sedimentary formations formed during marine transgressions from the Devonian through the Pennsylvanian periods
• Sauk Sequence - A group of North American sedimentary formations formed during marine transgressions in the late Proterozoic and Ordovician periods
• Tippecanoe Sequence - A group of North American sedimentary formations formed during marine transgressions in the Ordovician, Silurian, and Devonian periods