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.
WHY ARE THERE SO MANY DIFFERENT TYPES OF SEDIMENTARY ROCK?
The type of sediment that is deposited is determined by a location’s position on the landscape. Consider a shoreline environment: dunes along the coast (or farther inland) and beach sands become sandstones or quartzites (metamorphosed sandstones); mud and silt are deposited farther out to sea and become mudstones, siltstones, and shales; calcite accumulates in shallow tropical waters, both precipitating from the water and from the deposition of sea creatures. Fewer sediments are deposited and preserved in the interior of continents, which is why most sedimentary rocks are from shores, deltas, or shallow marine environments.
For different types of sediment to overlie each other, the shoreline’s location must keep shifting. Many factors lead to never-ending movement in the position of the shoreline, including continuous variation in the strength of the sun’s radiation and consequent changes in the amount of the Earth’s water stored as ice. Over hundreds of thousands to millions of years, a single location will experience different sedimentary environments—a history preserved as consecutive layers of sedimentary rock.
Breakup of supercontinent Rodinia, 750 million to 525 million years ago: By 750 million years ago Rodinia was beginning to be pulled apart, as Antarctica and Australia headed westward. As the continents were separated, a series of large faults formed in the Grand Canyon region, including the Bright Angel Fault. These were normal faults, which form as a region is stretched and expanded, and result in some blocks of rock being dropped downward. The formation of these faults caused the Grand Canyon Supergroup strata to be tilted and blocks of Grand Canyon Supergroup rocks to be “dropped into” the basement rocks.
The period of breakup and faulting was also one of erosion and thousands of feet of sediment were removed from the landscape, including most of the Grand Canyon Supergroup strata and some depth of the basement rocks. These changes created the Great Unconformity, the boundary between the Grand Canyon Supergroup and the overlying Paleozoic sedimentary rocks.
Today the only Grand Canyon Supergroup strata that are preserved in the vicinity of the Bright Angel and South Kaibab trails are those on a down-dropped block below the Tipoff, termed Cremation Graben (German for “grave”). And even here, only the three lowermost strata, the Bass Limestone, Hakatai Shale, and Shinumo Quartzite are preserved.
GEOLOGY DETERMINES WHERE THE TRAILS ARE LOCATED
If you visited vista points on the South Rim before embarking on your hike, you likely stared at the steep Kaibab Formation that forms a 350-foot-high cliff just about everywhere, providing few locations to descend below the rim. The Coconino Sandstone and Redwall Limestone form similarly impenetrable barriers. The Bright Angel Trail follows the Bright Angel Fault: Movement along the fault broke the solid rock, allowing erosion to proceed more rapidly. Eventually steep talus piles formed along the fault zone, allowing passage through otherwise vertical cliffs. In addition, the faulting has caused the rock on the southeast side of the fault to be about 200 feet lower than that on the northwest side, and the trail can snake back and forth across the fault depending on which side provides easier passage. The benefit of the fault scarp is especially visible where the Bright Angel Trail cuts through the Coconino Sandstone above the 1.5-Mile Resthouse and along Jacobs Ladder below the 3-Mile Resthouse. This fault first formed during the breakup of Rodinia 750 million years ago.
In contrast, the South Kaibab Trail is predominantly a ridge route that exploits locations where the normally steep rock layers have begun to erode, because they are outcropping along a narrow ridge. And here too, the passage through the Redwall Limestone follows a small fault.
Passive margin, 525 million to 320 million years ago: The breakup of Rodinia created a “passive margin,” or tectonically quiet region, along the then western edge of North America, the Colorado Plateau region. Along such margins the seafloor often sinks rapidly continually creating more space for sediment to accumulate and allowing thick rock strata to form. The prominent striped layers in the Grand Canyon, a series of strata 4000 thousand feet thick, were deposited during this tectonic regime with the type of sediment changing with shoreline position and water depth. (Up to 18,000 feet of sediment was deposited on the Colorado Plateau, but only the lower strata are preserved in the Grand Canyon.)
The oldest (and lowest) of the layers is the Tapeats Sandstone, formed from former beach sands. The Bright Angel Shale, formed in shallow offshore waters, follows; the mud-sized particles comprising the shale were carried a short distance out to sea before being deposited. The Muav Limestone, the third layer, is constituted of a combination of calcite that precipitated and calcite-bearing shells that were deposited. Together, these three layers represent a 20-million-year period of rising sea level: While sand-sized sediment was amassing at one location, silt was being deposited in shallow water to the west, and calcite was accumulating even farther west. As the sea level rose, each environment shifted eastward, such that the three sediment types overlay each other in the Grand Canyon.
No records remain from the following 120 million years, probably because decreasing sea level at the end of this period allowed the upper sediment layers to erode. One intermediate layer, the Temple Butte Limestone, is present as a thick stratum in the western Grand Canyon, where waters were deeper, but along the Bright Angel and South Kaibab trails it exists only in eroded channels in the Muav Limestone. Note that each of the Grand Canyon’s rock layers above the Muav Limestone is separated by an unconformity; some gaps in the rock record are brief, but others correspond to the removal of considerable sediment.
When rock strata abutting one another do not represent a continuous time sequence, the surface between the two layers is referred to as an unconformity. This gap in time indicates that sediment was eroded from atop the lower stratum before the upper stratum was deposited.
By 340 million years ago sea level was again rising, and much of the Colorado Plateau region was submerged beneath a large, shallow sea. Rivers transported little sediment to the region, creating the clear water environment that promoted the deposition of the thick layer known as the Redwall Limestone. The sea then retreated, eroding the top of this layer.
Passive margin, but tectonic
