America's Volcanic Past -
Indiana

Indiana is a large anticline that plunges to the northwest. Consequently, the age and type of rocks in Indiana are governed by this large structural feature. The youngest rocks are in the northeastern and southwestern corners of the state, and the oldest are in the southeastern corner. The oldest rocks are primarily limestone, dolostones, and shales, whereas the youngest rocks are mostly sandstones and shales with minor amounts of limestone and coal. The distribution of rock types is the major control on the physiographic provinces in the southcentral part of the state.

Indiana's Rocks:
There is a large variety of rocks that are found in Indiana. All the rocks that are exposed at the bedrock surface, however, are sedimentary rocks. Most consist of sandstone, shale, siltstone, limestone, and dolostone. Other rock types are coal, conglomerate, gypsum, claystone, and chert. Deep wells and exploratory test drillings have encountered granite, gabbro, basalt, andesite, and metasedimentary rocks at depths of 3,500 feet to about 3 miles below the bedrock surface.

The Interior Plains:³
The Interior Plains is a vast region that spreads across the stable core (craton) of North America. This area had formed when several small continents collided and welded together well over a billion years ago, during the Precambrian. Precambrian metamorphic and igneous rocks now form the basement of the Interior Plains and make up the stable nucleus of North America. With the exception of the Black Hills of South Dakota, the entire region has low relief, reflecting more than 500 million years of relative tectonic stability.

Indiana's "Basement"²
Rocks a billion or more years old lie deeply buried beneath the surface of Indiana. Because younger rocks rest on them, geologists call these rocks the Basement. Only about 20 drill holes in Indiana have reached these rocks. From these holes and from seismic soundings geologists have gotten a glimpse of the geologic history representing most of the time that has elapsed since the earth was formed. And they have learned that the Basement is only about 3,000 feet below the surface in east-central Indiana, more than 2 miles deep in southwestern Indiana. Samples of granite, basalt, and marble from the deep holes differ greatly from the sedimentary bedrock on which we live and from which we get most of our mineral resources. Knowledge of these rocks is important because similar rocks elsewhere contain rich mineral deposits. And their position in some places in Indiana controls the structure of overlying rocks that may contain gas and oil.

Precambrian History:¹
Known Precambrian history in this area began with the emplacement of a vast, horizontal, 7-mile-thick layered sheet of granite (coarse-grained igneous rock formed at depth) and rhyolite (fine-grained volcanic equivalent of granite formed near the surface) beneath western Ohio and neighboring states to the west. This emplacement has been attributed to an uprising in the Earth's mantle, known as a superswell. Radioisotopic dating suggests that this event took place between about 1.4 and 1.5 billion years ago, forming what geologists call the Granite-Rhyolite Province. Continued continental doming of the superswell caused the crust beneath western Ohio, Indiana, and Kentucky to extend and split (rifting), resulting in major faulting and consequent downdropping to form a complex rift basin, now known as the East Continent Rift Basin. Molten basalt flowed upward as erosion began to fill the basin with clastic sediment, perhaps as much as 20,000 feet thick in some places. This extensive deposit is known as the Middle Run Formation. About 1 billion years ago, doming ceased and the rift became a failed or aborted rift. Rifting, volcanic activity, and basin filling also ceased.

North American Craton:²
Indiana lies toward the middle of the North American continent, far from the coastal regions and mountains. Continents move about the surface of the earth, bumping into other continents and drifting away. This takes millions of years to happen, but the results can be seen today in the rocks of the continents. The middle of most continents has not been crunched or squished up the way the edges have been. That is because the middle of the continents are made of very strong old rock. This part of the continent is called the stable craton. Indiana is located near the middle of the North American craton. While much of the stable craton is exposed at the surface north of Indiana as the Canadian Shield, the middle of the craton, located in the United States, is covered with sedimentary rocks of the Interior Platform. The sequence of rocks varies from approximately 3,500 in excess of 20,000 feet in thickness. The cratonic rocks are metamorphic and igneous while the overlying sedimentary rocks are composed mostly of limestones, sandstones, and shales. These sedimentary rocks were deposited from 650 to 290 million years ago. The metamorphic and igneous rocks of the "basement complex" were created 1.5 to 1.0 billion years ago in a tectonically active setting. We don't know much about this setting but by observing the textures and compositions of these rocks we know that it was a setting of great pressure and temperature. The younger sedimentary rocks that were deposited on top of this basement complex were formed in a setting of quiet marine and river waters. Much of this time the craton was covered by a large shallow sea, a so-called "epicratonic sea" (meaning literally "on" the craton). Sometimes land masses or mountain chains rose up on the distant edges of the craton and then eroded down, shedding their sand across Indiana. These sediments whether deposited in rivers or seas eventually compacted into the sedimentary rocks that cover the state today.