Precambrian Beginnings: Roots of the South Central

The Earth is estimated to be approximately 4.6 billion years old. Rocks dating to 4 billion years old are found on almost every continent, but the oldest rocks known are 4.3-billion-year-old green-stone beds found along the eastern shore of Hudson Bay in northern Quebec. These are part of the Canadian shield, the ancient core of the North American continental landmass.

The shape and position of North America has changed dramatically over the last billion years, and geologic processes continue these changes today. Compression from colliding plates, tension from plates pulling apart, the addition of land to North America, weathering, and erosion have combined to slowly sculpt the form of the continent. More than a billion years ago, narrow strips of land collided with and compressed one another to form the Precambrian beginnings of the North American continent, also called Laurentia. From this proto-North America, sediment was eroded and transported by rivers and streams across the ancient continental margins and then into the adjacent oceans. The sediment deposited in the ocean waters on the eastern margin of Laurentia composes what is presently called the Grenville Belt.

Sedimentary Structures

Sedimentary rocks often reveal the type of environment in which they formed by the presence of structures within the rock. Sedimentary structures include ripple marks, cross-beds, mud cracks, and even raindrop impressions. Consider the type of environments in which you see these sedimentary structures today in the world around you.

Ripple marks suggest the presence of moving water (though wind can also create ripples and even dunes). Mud cracks indicate that the sediment was wet but exposed to the air so that it dried and cracked.

Cross-beds form as flowing water or wind pushes sediment downcurrent, creating thin beds that slope gently in the direction of the flow as migrating ripples. The downstream slope of the ripple may be preserved as a thin layer dipping in the direction of the current, across the natural flat-lying repose of the beds. Another migrating ripple will form an additional layer on top of the previous one.

The South Central States are divided up into five different geologic provinces or regions (Figure 1.3): the Central Lowland (1), Interior Highlands (2), Coastal Plain (3), Great Plains (4), and the Basin and Range (5). Among these regions, Precambrian-aged rock is exposed in only a few discrete places in Texas, Oklahoma, and Missouri, although it underlies larger swaths of the area. Much of the basement rock here is a remnant of volcanic islands and an associated submarine mountain range that formed about 1.4 billion years ago as magma welled up from the mantle and cooled into granite or erupted, forming rhyolite. These islands later collided with other microcontinents to form the bedrock of the proto-North American continent.

The oldest exposures in the South Central are found within the Saint Francois Mountains of the Interior Highlands. Granitic and rhyolitic igneous rocks represent the remains of a Proterozoic mountain range dating to nearly 1.5 billion years ago, and they form the core of the Highlands today. Further south, the Central Texas Uplift of the Great Plains province contains gneiss and granite that formed between 1.3 and 1 million years ago. The Franklin Mountains near El Paso, Texas, part of the Basin and Range province, also contain Precambrian-aged rock.

Figure 1.3: Geologic regions of the South Central.

During the next several tens of millions of years, parts of these mountains were buried in sediment that originated on Laurentia’s northeastern portion. The resulting sedimentary rock was subsequently metamorphosed, and it is now exposed at the Llano Uplift in a ring surrounding the previously mentioned older portion.

The remainder of the Precambrian period saw the formation of the supercontinent Rodinia about 1.1 billion years ago (Figure 1.4), along with its eventual breakup about 750 million years ago. The tectonic activity that generated the formation and breakup of Rodinia may have contributed to the metamorphism evident in the older rocks mentioned above. As Rodinia broke up, a rift began to form in what is now southwest Oklahoma. Magma rising near the surface cooled to form parts of the Wichita Mountains. The rifting eventually failed, but not before the crust had thinned sufficiently to create a series of inland basins in southwestern Oklahoma and western Texas.

By the close of the Precambrian, very little existed of the South Central as we know it today. Deposition of sediments within shallow seas during the Paleozoic and Mesozoic, followed by deposition from the eroding Rocky Mountains and Interior Highlands during the Cenozoic era, would later form the South Central familiar to geologists today.