3.7: The Story of Plate Tectonics as Recorded by the Rocks

Introduction

While the picture of early plate tectonics is still fuzzy, more recent rocks paint a clearer one. As mentioned elsewhere in this text, rocks record the history of the Earth. Sedimentary rocks are especially useful because their "pages" record the geologic and biologic activity happening on Earth's surface at the time they were made. The Appalachian region of the United States east coast provides an excellent record of nearly the entire Paleozoic Era in its rock record. A drive along Corridor H (US 48) through eastern West Virginia will start in the Cambrian carbonate strata and end in Carboniferous coals and sands. During this time, the North American craton would see the opening and closing of the proto-Atlantic Ocean (Iapetus Ocean), plate movement from southern subtropical latitudes to tropical latitudes, and the coming and going of large interior seas. The rock record in this region is diverse and important. The ebb and flow of sea level and facies have left a very detailed record of changes over time.

Sedimentary Sequences of the Paleozoic

Four large sequences of sea level transgression took place around the North American craton from the late Proterozoic Era through the end of the Paleozoic Era. The oldest, the Sauk Sequence, is dominated by carbonate rock formations. The youngest two, the Kaskaskia and Absaroka, record the closing of the Iapetus Ocean Basin. In between these and the Sauk exists the Tippecanoe Sequence. Based upon sediment thickness and volume alone, it is likely that this transgression was the deepest of all of them in terms of water depth. It also records the evolution of the ancient Taconian Mountains.

Figure \(\PageIndex{1}\): Sedimentary “Sloss” Sequences of North America. In the Mid-Atlantic region, the Sauk, Tippecanoe, and Kaskaskia are well exposed. White and yellow areas represent major marine transgressions onto the craton while black areas represent major unconformities (sequence boundaries). The Sauk is bounded on the bottom by the “Great Unconformity”, the boundary between the Sauk and Tippecanoe is referred to as the “Knox Unconformity”, and the boundary between the Tippecanoe and Kaskaskia is the “Wallbridge Unconformity”. All six of Sloss’ Sequences are 2nd order supercycles. (Sloss, 1964)

In 1963, a geologist named Lawrence Sloss from Northwestern University became famous because of a paper in which he identified six sequences of sedimentation that occurred along the margins and interior of ancient North America, known as Laurasia, from the late Neoproterozoic through the early Cenozoic Eras, a time span of close to a billion years. These sequences were given names. The oldest was the Sauk, followed by the Tippecanoe, Kaskaskia, Absaroka, Zuni, and Tejas. The names of these have their various origins, but geological names are always linked to a physical location. This sequence of rock layers records the history of this sea at that location. As the Tippecanoe Sea surrounded all of Laurasia, there are many places in North America where you can see a progression of this sequence in the stratigraphic record, including, for example, the Grand Canyon.

Figure \(\PageIndex{2}\): Cratonic Sequences, or Sloss Sequences, as they exist in the Colorado Plateau and Grand Canyon region in the American southwest.

Sloss (1964) identified these sequences through the development of curves based on data on land subsidence (lowering), cratonic interior uplift, and sedimentation. The curve explores global (eustatic) sea level changes related to base level. Each Supersequence, Sauk, Tippecanoe, etc., is similar. The sequence begins with high eustatic sea levels above the base level, progresses through a time of much lower sea levels, and then ends with a major transgression of the sea to end the sequence. Each sequence begins with a sequence boundary, a regression, and a coastline progradation (coastline extending further out to sea) and then ends with a major transgression. In the middle of all of these sequences are major orogenic (mountain-building) events.

Figure \(\PageIndex{3}\): Four plots depicting relative tectonic activity across ancient North America. From the top down, Sedimentation fluctuations, Yoked basin activity, Relative land subsidence, and Relative movement of the cratonic interior. Tectonic, sedimentation, and global (eustatic) sea level activity would contribute to the deposition of genetically-related strata being deposited over the course of six periods of time as sequences of sediment packages separated by erosional sequence boundaries. (From Sloss (1964).)

Watch this great two-minute video that puts together the Sloss Supersequences with the tectonic and environmental changes taking place in North America. Pay attention to the time periods in the Paleogeographic images. The Sloss Supersequence diagram is much clearer if you watch it in full screen however, you can match the diagram in the video with the diagram above for better clarity.

The Cycle of Plate Tectonics - The Wilson Cycle

Four of these six sequences (Sauk through Absaroka), all were deposited during one very long 1st order megasequence. This megasequence marks the opening and closing of the Iapetus (ancient Atlantic Ocean) as a whole. It is also known as a “Wilson Cycle”. The Wilson Cycle, named after the Canadian geophysicist J. Tuzo Wilson, is a model that explains how an ocean basin opens and closes, beginning with a divergent boundary and ending with collisional subduction between two continents. In his seminal work (1966), Wilson built upon earlier observations of fossil similarities existing on either side of the Atlantic by proposing that there also existed a “proto-Atlantic Ocean” between the continents during the Paleozoic Era. This ocean closed by stages, bringing dissimilar rocks together as the supercontinent Pangaea formed. This simple model can then be applied to other contexts, such as the closing of the Tethys Sea as India moved northward toward Asia, etc. Watch this simple one-minute schematic video of the Wilson Cycle below.

Stages of the Wilson Cycle

There are nine stages to a Wilson Cycle, 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.

Closing of the 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 Tippecanoe Sequence takes place in this model between stages E through G.

Figure \(\PageIndex{4}\): Stages A through E of a Wilson Cycle (Whitmeyer et al., 2007)

Figure \(\PageIndex{5}\): Stages F through I of a Wilson Cycle (Whitmeyer et al., 2007).

The Tippecanoe Sequence begins with missing rock, a disconformity. Called the Knox Unconformity, it serves as the sequence boundary across the craton. It can be seen in the strata of the Grand Canyon as well as in the strata of western Virginia. It is an erosional surface resulting from a rapid drop in sea level. Throughout the Tippecanoe, sea level is transgressing onto the craton. As average sea level rose, there were fits and starts of smaller sequences, higher order sequences, within the Tippecanoe. These are due to a variety of factors. The story of the Tippecanoe includes Bahamian-like offshore reefs and tidal expanses, extensive muddy submarine landslides, sandy beaches, and a return to carbonates. It includes volcanic ash from Taconian eruptions and various important body fossils.

Key Terms

• regression - a time period when sea levels drop relative to land masses; caused by sea level drop or land level rise
• transgression - a time period when seas spread over land areas; caused by sea level rise or land level drop