Celebration of Scholarship and Creative Activity 2023
The Earth has gone through several major ice ages. One of the most extreme ice ages occurred during the Cryogenian period (650-630 Ma) and is recorded in rocks of the Jacadigo Basin, Brazil. This was an important transition period in Earth history in which oxygen levels rose and allowed for the evolution of animals. Sedimentary rocks record evidence of this and other ice ages, and among these records is sediment dropped from melting ice (dropstones). The dropped sediment includes fragments of bedrock from the area and glacial till. This provides important information about flow direction of glacial ice, the composition of the area, and climate cycles. Over the 600-million-year history of these rocks, alteration oftentimes obscurs the texture. We used microscopic petrography and X-Ray diffraction analyses to recognize mineral composition and alteration. At this scale, clear implications of post-depositional alteration are visible. Additionally, differing composition and grain size, as well as the crystal growth in the surrounding matrix, indicate whether material was a dropstone or not. Dropstones from this particular area reflected a granitic composition as they are predominantly composed of potassium feldspar, plagioclase feldspar, micas, and quartz. Additionally, fragments of glacial till that contain granitic sand and pebbles are excellent evidence of the ice-age connection. The glacial dropstones occur in rocks that represent a variety of glacial marine settings, but predominantly occur in layers marked by microbially laminated textures. This suggests they fell to a seafloor that was covered by glacial ice.