Integrative Natural History of Amnicon Falls State Park, Part 3: The Douglas Fault Exposed | Wisconsin, USA
(Updated April 16, 2025)
Looking east-southeastward at the eastern cliff cut by the middle branch of the Amnicon River. The Upper Falls, the focal point of Part 2 of this set, is just out of view to the right.
It's always a treat when concepts of structural geology are glaringly visible at the Earth's surface. Here an absolutely lovely fault, the Douglas, runs up that cliff from lower right to upper left in the center of the frame. If you stand on the little enclosed observation platform, you're right next to it.
The crustal section on the right, massive, dark-brown basalt of the late-Mesoproterozoic Chengwatana Volcanic Group, constitutes the hanging wall. The steps and the platform, and everything below and to their left, are the somewhat younger (probably very-late-Mesoproterozoic) Orienta Sandstone. It's the lowest formation of Wisconsin's Bayfield Group. Here it's the footwall.
When a portion of the Earth's crust is pulled apart—when it's under tension—normal faults, with down-dropped hanging walls, are produced. But when you see the opposite condition, where the hanging wall has been pushed up the fault plane at a high angle, it's a reverse fault instead. This indicates crustal compression.
As it so happens, the Douglas and some other major faults associated with the Midcontinent Rift (MCR) have been both types. When the MCR first developed during a phase of regional stretching, they came into being as the normal variety. But then, at some point afterward, the MCR structure was subjected to crustal scrunching, and they were converted into the reverse kind. Their headwalls were displaced upward. So in this spot the older Chengwatana basalt sits atop the Orienta Sandstone.
As a photo in a future posting will show, the contact between the two rock types contains broken-up, ground-up, and generally messed-up rock that suffered the worst of the frictional effects when the fault moved. Geologists of my ancient generation refer to this by the informal name of fubarite, but the official terms are fault gouge (finer-grained material) and fault breccia (with larger clasts).
At this distance you can see how the usually flat-lying beds of the Orienta have been seriously deformed and are dipping at crazy angles. If you walk downstream from here, however, you'll see that the strata are still in their original, more-or-less horizontal orientation.
You'll find the other photos and descriptions of this series in my Integrative Natural History of Amnicon Falls State Park album.
Integrative Natural History of Amnicon Falls State Park, Part 3: The Douglas Fault Exposed | Wisconsin, USA
(Updated April 16, 2025)
Looking east-southeastward at the eastern cliff cut by the middle branch of the Amnicon River. The Upper Falls, the focal point of Part 2 of this set, is just out of view to the right.
It's always a treat when concepts of structural geology are glaringly visible at the Earth's surface. Here an absolutely lovely fault, the Douglas, runs up that cliff from lower right to upper left in the center of the frame. If you stand on the little enclosed observation platform, you're right next to it.
The crustal section on the right, massive, dark-brown basalt of the late-Mesoproterozoic Chengwatana Volcanic Group, constitutes the hanging wall. The steps and the platform, and everything below and to their left, are the somewhat younger (probably very-late-Mesoproterozoic) Orienta Sandstone. It's the lowest formation of Wisconsin's Bayfield Group. Here it's the footwall.
When a portion of the Earth's crust is pulled apart—when it's under tension—normal faults, with down-dropped hanging walls, are produced. But when you see the opposite condition, where the hanging wall has been pushed up the fault plane at a high angle, it's a reverse fault instead. This indicates crustal compression.
As it so happens, the Douglas and some other major faults associated with the Midcontinent Rift (MCR) have been both types. When the MCR first developed during a phase of regional stretching, they came into being as the normal variety. But then, at some point afterward, the MCR structure was subjected to crustal scrunching, and they were converted into the reverse kind. Their headwalls were displaced upward. So in this spot the older Chengwatana basalt sits atop the Orienta Sandstone.
As a photo in a future posting will show, the contact between the two rock types contains broken-up, ground-up, and generally messed-up rock that suffered the worst of the frictional effects when the fault moved. Geologists of my ancient generation refer to this by the informal name of fubarite, but the official terms are fault gouge (finer-grained material) and fault breccia (with larger clasts).
At this distance you can see how the usually flat-lying beds of the Orienta have been seriously deformed and are dipping at crazy angles. If you walk downstream from here, however, you'll see that the strata are still in their original, more-or-less horizontal orientation.
You'll find the other photos and descriptions of this series in my Integrative Natural History of Amnicon Falls State Park album.