Читать книгу One Best Hike: Grand Canyon. Everything You Need to Know to Successfully Hike from the Rim to the River—and Back онлайн
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Formation and existence of supercontinent Rodinia, 1.2 billion to 750 million years ago: While the Grand Canyon was experiencing a period of tectonic calm and associated erosion, the global stage was being set for a set of massive collisions that formed the supercontinent Rodinia. Rodinia incorporated most of the Earth’s land masses. Australia and Antarctica were welded onto the western edge of the North American continent, west of the Grand Canyon. Along the eastern edge of North America, the Grenville Orogeny, beginning 1.1 billion years ago, created the mountains of the eastern seaboard and apparently caused the western edge of North America to tip downward, creating a narrow sea at the border of North America and the Australian/Antarctic landmasses.
When sedimentary rocks are horizontally layered, the layers indicate the order in which the sediment was deposited—the oldest sedimentary layer is at the bottom and the youngest on top.
The Grand Canyon region was now a costal environment and the sediment that comprises the Grand Canyon Supergroup began to be deposited. Initially the shallow Bass Sea covered the area, depositing the calcareous sediment that constitutes the Bass Limestone. Subsequently, a decrease in water level led to the deposition of mud atop the limestone, creating the Hakatai Shale. Additional decrease in water level caused beach sands to be deposited, coalescing into the very erosion-resistant Shinumo Quartzite. What followed were small-scale encroachments and retreats of the sea, creating bedding shales and sandstones, the Dox Formation. The final formation in the first group of Supergroup rocks is the Cardenas Lava, dating to 1.1 billion years ago, coinciding with the Grenville Orogeny and small-scale rifting in the Grand Canyon region. These first five formations are collectively known as the Unkar Group. The remaining Supergroup formations were deposited in the sea deep within Rodinia. Since they outcrop on neither the South Kaibab nor the Bright Angel trails, they are not described here.