General Information about Breccia Processes
Sedimentary Rocks in Devonian
Upon impact, rock fragments are separated from the seafloor. This separation boundary represents the detachment surface. The depth of the detachment surface depends upon the size of the meteor, its speed, and the angle at which it hits.
In terms of the Alamo Impact, the detachment surface cuts through the limestones of the Ledge-forming interval (or LFI). Going westward towards the crater, the detachment cuts deeper into the LFI. This surface marks the base of the Alamo Breccia, recognized in the field by the presence of Breccia D.
When a meteor strikes the ocean an incredible amount of energy is released, fragmenting the seafloor and forming massive waves. The debris is carried by a surge of waves away from the area of impact (stage 2). As the waves dissipate, the heavier debris fall down and are deposited along the seafloor as surge deposits (stage 3). Surge deposits often contain very large rock fragments mixed with smaller fragments.
Resurge takes place following the initial surge. After a large volume of water is moved away from the crater (towards the shore) during the surge, water rushes back offshore and creates a resurge of waves towards the crater (stage 4). These waves pick up and re-deposit the rock fragments from the initial surge. The surge and resurge waves occur simultaneously and dissipate the wave energy (stage 5). Resurge deposits are better sorted than surge deposits and contain smaller rock fragments.
As wave energy begins to dissipate following the surge and resurge, suspended sediment settles out of the water and onto the seafloor. Normal-grading is a common feature of deposits formed by settling processes. Larger grains that require more energy to remain suspended settle out first. Followed by smaller and smaller grains as energy lessens. Once the energy created by the meteor impact has completely dissipated, normal seafloor deposition resumes. This marks the end of the impact event deposits.
Upon impact of the Alamo bolide, some of the fragmented rocks, called ejecta, were thrown out of the water and up into the air at very high speeds. The impact force was so large that an entire wall of ejecta material was created and pushed away from the crater in all directions. This wall of material is termed an ejecta blanket.
Ejecta material can be transported and deposited several miles away from the crater, depending upon the size and force of impact. Ejecta sizes range from very large, boulder-sized fragments to smaller, sand-sized ones. Impact accretionary lapilli are commonly found within impact ejecta material. Heat generated from the impact force melts rock as it blasts fragments into the air as ejecta. Some of the melted rock accretes around the blasted fragments, cools, and forms jawbreaker-like balls. Much like hailstones add ice when falling through the clouds, the lapilli increase in size by accreting melted rock as they fall to the ground. This accretion process forms a concentric ring pattern that can be seen in cross-section.