Stratigraphy

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Section of rock types in Bisbee

Lowell Volcanics  not shown

 
 

Pre-Cretaceous 

 
 

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Pinal schist is the Precambrian basement unit in Bisbee., which has been dated to 1.7 billion years old. Fine grained light to gray colored sometimes greenish, probably was arkose sand and slit before metamorphism. Separated from the Bolsa quartzite by a well-defined unconformity.

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Bolsa Quartzite is 430 feet thick, with pebbly beds at the bottom with it progressively becoming fine-grained at top of the unit. These features and crossbedding show the depositing sea started shallow and got deeper as time went on.  The age is estimated to be lower to middle Cambrian, no fossils have been found in the formation. 

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Abrigo limestone is 770 feet thick, thin-bedded, and a shaley unit. Middle to upper Cambrian in age, Bisbee's first appearance of marine fossils occurs in this formation. The limestone is named for Abrigo canyon which is 3 miles southwest of Bisbee.

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Martin limestone varies from 300 to 375 feet thick, mixture of shaley and crystalline beds.  Devonian in age, this unit is very important for determining structure because of its very discernable marker beds. Deposited in open ocean, some distance from shore indicated by lack of dolomite, terrigenous sediments and it contains abundant fossils. Some of the fossil are silicified and weather out very distinct .

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Escabrosa limestone  varies from 600 to 800 feet thick, thick bedded white to dark gray in color and granular in texture. Large crinoid fragment dispersed through the unit give it the graininess. Mississippian in age, believed to have deposited 30 to 40 miles from shore and at depth of 600 feet.

Naco Group

 

 The former Naco Limestone, now has been divided into six formations of which the Horquilla, Earp and Colina are found in Bisbee.  
 

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Naco limestone   Mostly made up of fairly pure light gray limestone originally thought to have been over 3000 feet thick, now eroded down current thickness of at least 1500 possibly 2000 feet. Deposited in a moderately deep ocean, though shallower  than the Escabrosa. This Pennsylvanian age unit is full of abundant fossils, dominated by brachiopods.  Now  turned into the Naco group see below for units represented in Bisbee.

 

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Horquillla limestone   the lowest section of the Naco group locally up to 1221 ft. thick . Colorization is thick pale red, pinkish gray to light gray in color, commonly fossiliferous containing Crinoid stems, Brachiopods and Bryozoans.  Deposited in a shallow sea, small amounts of terrigenous sediments indicate possibly an island landmass was nearby.
 

Earp Formation    is the middle section of the Naco group rocks found at Bisbee. Up to 595 ft. thick, mostly made up of siltstone, limestone with minor amounts of sandstone and mudstone. Formed in a shallow sea contains terrigenous sediments, with the unit thickening to the north, which implies a landmass was located in that direction..

Colina limestone   495ft of thickness has been measured in the Naco hills. The unit is generally thick bedded, resistant to erosion, light medium gray color when weathered. Abundant fossils are found in this unit which formed in a shallow sea environment.

Bisbee Group 

A series formations containing  Cretaceous age sediments deposited by the Bisbee -Mancos Sea. 
 

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Glance Conglomerate  Cretaceous age unit that lies from 50 to 653 feet thick usually reddish in color. The unit is made up of coarse poorly rounded fragments of the pre-Cretaceous rocks. It is thought to have been deposited along a marine shore line the material was rapidly submerged due to the land subsidence and quickly buried by fine gravels muds of the Morita formation
 

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Morita Formation   1800 feet of this unit is exposed in Bisbee, with it getting thicker as it goes south. The unit is made up of dull red shales and red to tan sandstones with lenses of impure limestone. Deposited in shallow water setting that is evolving from a beach to a moderately deep water environment. land  was located to the west of Bisbee at this time.

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Mural limestone  650 feet thick with some beds made up of almost entirely of oyster shells. The upper 350 feet  is a gray colored, thick bedded unit that forms the cliffs that are one of Bisbee most recognizable geologic features, easily seen in the hills behind the Denn mine. The deposit started  with sand and muddy silts coming from a land mass to deeper and cleaner rocks mostly formed from marine organisms.

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Cintura Formation   original thickness unknown due to quaternary erosion, 1800 feet exist today made up of purplish shales some gray limestone beads and tan sandstones. Some fossil turritella found in the limestone sections Deposition environment  returned to similar to morita formation, beaches and moderate deep oceans. This thought to have been caused by uplift of the local lands mass causing an influx of land sediments.

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Volcanic Rocks 

 
 

Juniper Flat Granite  Intrusive volcanic rock dating  from 170 to 185 ma.  Red –gray color mostly made up of orthoclase. The best exposure is northeast of Bisbee  were it makes dramatic cliffs original 

Lowell Volcanics   It is made up of  tuff and believed to be Jurassic in age. The unit has limited exposure and is located in the hanging wall of the Divided fault along highway 80 and  in a small area below the Glance conglomerate   inside the Lavender Pit.   

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