The Abrigo Limestone

The Abrigo Limestone is middle to upper Cambrian in age, and is found in southeastern Arizona. This forms a sequence with the Bolsa Quartzite and the Bosla equivalent in Greenlee county, the Coronado Quartzite of a marine transgressive (Bolsa and Coronado) and a regressive (Abrigo) deposition cycle of a shallow marine sea. Over 800 feet thick at some localities, the Abrigo Limestone contains locally abundant marine fossils, including primitive crinoids, dozens of species of trilobites, hyalithids, several ichnospecies of trace fossils, and lingulid brachiopods. The Abrigo consists of a dark shaley lower member, a middle member, an upper sandy member and finally the uppermost Copper Queen member that spans in time into the Trempealeauan and Franconian stages of the upper most Cambrian. The Abrigo formation is chronologically equivalent to the Bright Angel Shale and Muav Limestone found in Northern Arizona. This locality near Colossal Cave in southern Arizona is now annexed into state park land, and is no longer accessible. We were very lucky to have the opportunity to explore here.


My wife Dawn moves slowly amongst the shales and limestones to not miss a single trilobite pygidium or cranidium! You can see the loosely bedded plates of limey shales eroding out. This is typical of most exposures of the Abrigo.

Friend Gordon Nelson, wife Dawn and friend Vince Mele show off the days finds of trilobite parts and hyalithids. We are standing in the Bolsa Quartzite, at the base of the outcrop.

The author in a lower green shaley unit (Bright Angel Shale equivalent) holding green shale trace fossils (paleophycus), and some large burrows with smaller ones superimposed on them.

Dawn in the unusual green shales. The whole hillside is covered with green slabs with countless worm traces (planolites and paleophycus) indicating that the green shales were once a muddy sea bottom, with a subsurface teaming with life full of soft bodied organisms. The shales also had a spectacular mica sheen to them, acquired during some post diagenetic stage.

Some of the lower beds of the middle member had sandy shales in them with easily visible cross bedding. A higher energy enviornment was responsible for this, showing much of the Abrigo in this area was very near shore.

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