First today, I don’t think I’ve given my usual disclaimer for a while, to point out that this calendar is not at a correct scale for the history of the earth. If it were, we’d be in the Precambrian until mid-November and the daily episodes would be pretty boring. So I’ve arbitrarily assigned the Precambrian to January, the periods of the Paleozoic to February through August, the Mesozoic periods to September, October, and November, and the Cenozoic Era gets December.
Second, as near as I can tell based on downloads, you the listener must be more or less enjoying the posts. But I’d love to hear feedback, whether through a review on iTunes, a comment on the blog, or in an email to email@example.com. And please feel free to ask questions – I have a couple that are pending, about the Late Heavy Bombardment, an update on work on the Belt Basin, and shallow versus deep-water limestones, and I hope to get to those soon.
So back to the Permian.
The last time we talked about the Grand Canyon, it was the Mississippian Redwall Limestone, pretty much the most prominent cliff-former in the canyon. It exemplifies the vast, shallow warm carbonate seas of the Mississippian that spread over much of what is now North America.
|Supai Group (National Park Service photo)|
The rocks above and younger than the Redwall are the Supai Group, diverse rocks that span time from Late Mississippian into the Permian. They include limestones, as well as non-marine rocks that were probably laid down in a muddy coastal plain setting. Some of the Pennsylvanian sediments were eroded off the Pennsylvanian Ancestral Rockies to the northeast of the Grand Canyon area. Especially in the upper part of the Supai Group, the sandstones and siltstones and shales tend to be reddish – an indication of oxidized iron, which is an indication that the environment was alternately under shallow water and exposed to the atmosphere. The rocks have amphibian footprints in them, further evidence of the coastal nature of these sediments.
Even higher in the stratigraphic section, the Lower Permian Hermit Formation overlies the Supai Group. The Hermit Formation is red and white sandstone and siltstone and especially bright red shale. Because the shale is relatively non-resistant, it erodes easily, and it’s the red material from the Hermit Shale as well as some of the rocks of the Supai Group that have stained the underlying Redwall red. The Redwall limestone is fundamentally a grayish rock, but the bright red color that gives it its name comes from iron oxide eroded out of overlying layers.
The rocks of the Hermit Formation show cross-bedding, angular layers within individual sandstone packages, which are produced by currents moving sand grains in one particular direction. Either wind or flowing water can make cross-beds. Wind-generated cross-beds often represent the faces of sand dunes. Water-borne moving sand can also form dune-like shapes underwater, and most of the Hermit cross-bedding is the result of channelized flow in river systems. The actual channel margins can also be found in the Hermit, where rivers cut into underlying strata as much as 130 feet deep – although channels of that extent are unusual. But the Hermit was laid down on an erosional surface, so there is a distinct unconformity at its base.
The abundance of red colors in the Pennsylvanian and Permian rocks of the Grand Canyon reflect the increasing arid conditions, which were global in extent. Instead of shallow seas, the planet was becoming dominated by land. This is partly a result of the ongoing continental collisions that were creating the supercontinent of Pangaea, and probably also a result of the increasing glaciers across Gondwana in the south polar region. Glaciers take up water, and lower sea level. We’ll be talking more about Pangaea and the glaciation throughout this month.
—Richard I. Gibson
USGS Hermit Formation
See also The Earth Story
Photo (public domain) from National Park Service.