The Importance of Breccia
Sedimentary Rocks in Devonian
The Alamo Breccia is one of the best-preserved impact-generated rock intervals on the Earth's surface. Today it is exposed throughout 25 mountain ranges across southern Nevada and western Utah.
The formation of the breccia was triggered by a bolide impact into the Late Devonian seafloor, consisting of limestone. The force of the impact carved through much of the underlying strata, creating limestone rock fragments that were driven outwards and re-deposited as the Alamo Breccia. Tsunami waves generated seconds after impact carried sediment away from the crater (surge) and back into the crater to fill the void (resurge).
The Alamo Breccia is split into several distinctive vertical facies, known as Breccias A, B, C, and D, that correspond to the variety of transport methods of impacted material after impact. The description of these breccia facies has allowed scientists to better understand the transport processes throughout the impacted region.
Another unique feature associated with the Alamo Breccia is the appearance of carbonate accretionary lapilli. Heat generated from the force of the impact melted carbonate rock that formed around other debris, resulting in jawbreaker-like balls with concentric rings in cross-section. Lapilli associated with the Alamo Impact are not widespread.
Breccia A is the topmost breccia facies characterized by a polymict breccia. The breccia consists mainly of large gravel and is normally graded. Of facies A through D, it is the most organized and universally present. The uppermost intervals are roughly 1 meter in thickness and grade from carbonate gravel and/or sand to mud. The return of marine life after the Alamo Impact is shown in the fossilized burrow tracks present within the upper part of Breccia A.
The Breccia A facies is interpreted to be the result of gravity-flow deposits from a complex series of tsunamis. On impact, initial surges of tsunamis are generated away from the crater and are followed by an inward collapse of the crater itself. The collapse generates resurges of tsunamis toward the crater, bringing back ejected material. This material is then deposited in the form of stacked graded beds. The upward grading, from coarse to fine, represents a change in transport energy during the series of tsunamis. Strong waves carried and deposited large grains, followed by waves of decreasing energy that deposited smaller and smaller grains over time.
Breccia B consists of a chaotic, polymict breccia. Unlike Breccia A, the rock fragments are mostly supported by mud. In outcrop, Breccia B lies on top of Breccia C and, in some places, squishes between the megaclasts of Breccia C. Material of Breccia B is a mixture of ejecta and resurge materials.
The Breccia C facies is made up of megaclasts that range up to 500 meters long and 80 meters thick. These are huge chunks of the seafloor bed that were blasted upon impact and cover Breccia D. The majority of the chunks are parallel to the underlying bedrock, and some show evidence of collision that peeled up the chunks and formed intraformational folds. Although previous studies have misinterpreted this facies as reef talus or fault breccia, it is clear now that it represents nearly half of the breccia linked to the Alamo Impact Event.
The Breccia D facies is less than 3-meters thick and consists of monomict breccia, defining the base of the Alamo Breccia. Breccia D extends for tens to hundreds of meters beneath the Breccia C facies.
Breccia D is interpreted as liquefied bedrock that was preserved between unbroken carbonate beds and Breccia C. Similar monomict breccia can be found around the Breccia C megaclasts and scattered throughout the Breccia B facies, areas where rock may have also liquefied.
The general definition of lapilli is any fragment of lava or rock that has been blasted into the air by a volcanic explosion or meteorite impact. The lapilli associated with the Alamo Impact are considered accretionary lapilli. Heat generated from the impact force melted carbonate rock and blasted fragments into the air. Some of the melted carbonate rock formed around the blasted fragments, cooled, and formed jawbreaker-like balls. Much like hailstones add ice when falling through the clouds, the lapilli increased in size by accreting carbonate as they fell to the ground. The carbonate accretion process forms a concentric ring pattern that can be seen in cross-section.