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THE EARLY GEOLOGICAL HISTORY OF THE BOISE BASIN

 

The early Mesozoic Cretaceous Period

250 to 65 million years ago.

The Boise Basin is located in the geographical confines of Boise County and is an area of mountainous terrain and diverse geological formations.

 The geological beginnings of the Boise Basin as we know it today began to develop in the late cretaceous, early tertiary periods or in other words 60 million to 140 million years ago.  The area has become a complex system of geological transformations since that time.  Cretaceous granodiorite underlies the basin and is part of the Idaho batholith. 

The Idaho batholith is an ancient structure perhaps hundreds of millions of years old.  Batholiths were created by the rising of molten magma to the surface of the planet from deep within the earth’s crust.  This material is the granite type rock that is found throughout the basin.  In a number of places and particularly in the Idaho City area there is evidence of porphyritic intrusions created during the Eocene epoch.  These structures occurred on top of the existing granite 30 million to 50 million years ago.

Within the earlier geological structures there are dike intrusions of igneous material which has infused itself into fissures ofthe underling material.  This is most noticeable as the dark formations running through the cliffs along the road cuts above Lucky Peak reservoir.  It was during and after this formation that the lode deposits were created by the embedding of metalliferous material within fissures along the dike intrusions.

 

 

                                                                                                                                                                                                                                                                                                (Photo by Author)

The early Cenozoic Tertiary Period

65 to 5 million years ago.

 Interestingly, substantial erosion occurred to these ancient and underlying structures.  Most of this occurred prior to the faulting, shifting and uplifting of the more recent structures that we now see as mountains and valleys.  While it may look like the streams and rivers of the area are responsible for the erosion and creation of these mountain and valley formations it was really the process of faulting and lifting of the early granite structure some 30 to 50 million years ago.  It was this tectonic movement that caused the Snake River plain to drop and the mountains to the north to rise.  The tops of the mountains in the basin area show evidence of having been part of the lower cretaceous granodiorite,pre-tectonic structures. 

 
 
 
   
                                        (Photo by Author)

These tectonic movements created the numerous fault zones that run through the basin at a north-east to south-west direction. This is where most of the stream flows occur to this day.  If you will notice, Mores Creek, Grimes Creek and many of the other drainages run a north-east to south-west direction.

The erosion caused by these early streams and rivers deposited much of the underlying gravels and sands that are found deep in the valley formations. These drainage systems continued to develop until later tectonic movements interrupted the flow of the Snake River system.  This created a huge lake which caused the flooding of the Snake River valley to a depth of nearly 4000 feet above the Boise area.  This flooding extended up the valleys of the Boise River and filled much of the basin. Much of the later clays and gravels were deposited across the basin during this time.  Later tectonic activity created a breach for the lake to drain and the drainage action of the original streams returned to their erosion processes. 

 

The later Cenozoic Tertiary Period

53 to 1 million years ago.

When driving Highway 21 from Boise to Idaho City it is hard to miss the basalt cliffs that line the route.  These geological formations were formed during the late tertiary period when lava poured through the fault underlying the creek bed.  This lava eruption blocked the stream flow causing the deposition of more gravel and other sedimentation.  These deposits became what are now known as bench gravels.  Eventually the stream eroded its way through the lava and continued the process of erosion until the creek is over a hundred feet below the basalt layer in some places .

 

 
 
 
                                     (Photo by Author)
 
 
As the lava filled the surrounding area it created a fairly flat layer as most fluids do.  As you look down at the reservoir when crossing the Mores Creek high bridge you will notice that the basalt layer is very thick and that the creek bottom is a great distance lower than the basalt layer.  This is due to millions of years of water flow eroding it's way through the basalt.  As the highway and the creek gain altititude the basalt layer gets thinner and the distance between the creek bottom and the basalt layer gets smaller.  Between the 30 and 31 mile marker on Highway 21 the basalt layer and the creek finally meet.  This is where the lava flow ended.  The creek hit the lava flow here and began it's erosion process through the basalt.  If you walk down to the creek on the south side of the highway just before the bridge you will see where the creek and the basalt meet.
 
 
 
                                      (Photo by Author)

 MODERN GEOLOGICAL CONDITIONS IN THE BASIN

 500,000 years ago to the present.

The basin as we know it today consists of forested mountains, grassy valleys and stream beds that vary from low gradient flatlands to narrow fast flowing drops.  Flat lands with meandering water flows are found in the Idaho City area, the Centerville area and along Canyon Creek and Grimes Creek as well as other smaller areas. 

As a result of these millions of years of geologic formations there are numerous rifts, faults, veins and placers that are filled with the minerals and ores of today.  The principal ores and minerals found in these deposits include among others, monazite, hematite, ilmenite, pyrites, antimony, silver and gold bearing quartz.  Many of these ores and minerals have eroded out of the veins and rifts in which they were formed to become the placers that lie in the streams of today.

It is difficult for some people to envision the Boise Basin as any kind of a basin at all.  Most people consider it to be just a forested area of mountains and valleys unlike any of the surrounding terrain.  Shouldn’t a basin be similar to some kind of dish?  What exactly makes this area a basin?  In looking at the basin today from any high vantage and using your imagination a bit, you will notice that this entire area is a large depression surrounded by higher mountains and ridges. 

The basin or depression is created by the Boise Ridge to the south and east including Lucky Peak, Deer Point, Shaffer Butte, Mores Mountain Hawley Mountain and the ridge line to the east of Horseshoe Bend to the Charter’s Mountains.  The northern boundary is the high ridge that separates the Payette River drainage from the Mores Creek drainage, including Mineral Mountain, Alder Creek Summit, Grimes Pass, Coulter Summit, Wilson Peak, Freeman Peak and Pilot Peak.  The eastern border includes Sunset Mountain and the low ridge of mountains running south between the Mores Creek drainage and the Boise River drainage.  The basin itself contains mountains and rises of no small size but nothing higher than the ridges around it.  That is today’s Boise Basin.

 
 
                      (Photo by M.E. Martin)

 Why do we care about all this boring stuff?

The reason for all this boring information if not obvious will be evident in later pages of this website.

 

 References:

Jones, E.L. Lode Mining in the Quartzburg and Grimes Pass Porphyry Belt, Boise Basin, Idaho.  Short paper, page 83 – 112 of Contributions to Economic Geology. Department of the Interior, United States Geological Survey, Bulletin 640. F.L. Ransome and Hoyt S. Gale, Geologist in Charge. Washington D.C. US Government Printing Office, 1916,

Lindgren, Waldemar. The Mining Districts of the Idaho Basin and the Boise Ridge, Idaho.   U.S. Geological Survey Eighteenth Annual Report. Washington D.C. US Government Printing Office, 1898.

Link, Paul K. Digital Atlas of Idaho. Idaho State University, Geosciences Department. 2002, http://lmnh.isu.edu/digitalatlas

Global Stratigraphic Chart.  International Union of Geological Sciences. Mississippi State University. 1998, http://www.msstate.edu/dept/geosciences

 
 
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