Today was our first day out in the field. We did some reconnaissance to check out the rocks and to see where to collect my samples. First, we hiked in (the trail runs somewhere behind this house).
Along the way we see a lot of rice paddies, corn fields, and sunflower beds.
There is a lot of vegetation to deal with – sometimes it is challenging to find good outcrop.
Below is a contact between two very different rock formations. The one on the left below the ferns is a fossiliferous limestone, while the one to the right is a very fine-grained carbonate, which weather more easily. The fossiliferous limestone is older than the fine-grained carbonate. This represents a relatively rapid shift in depositional environment (also known as ‘facies’) from a shallow water facies, which supports a lot of carbonate-producing biological activity, to a deep water facies. The deep water environment cannot support a lot of life, and therefore carbonate fossils, which are common on shallow shelf environments, are rare and the rock is made up of very fine grained material that has fallen down slowly in the water column. In addition to the fine silt and clay that has settled into this deep area, siliceous microscopic organisms known as radiolarians are deposited into a hard rock known as chert. Radiolarians are an example of a pelagic organism, meaning that they live in open water at the top of the water column. While not very visible in this photo, chert bands are common in this deep water rock formation.
Note that at this time, carbonate pelagic organisms that are common in modern oceans, such as pelagic forams and coccoliths had not yet evolved.
Later in the day, we explored another rock formation that consists of carbonate breccias alternating with thinly bedded limestones. Below, the breccia can be seen in the large gray rock in the middle of the photo, dotted with lighter gray pieces of rock. A breccia is a type of sedimentary rock that consists of large clasts (or grains) that have been deposited through high energy processes such as a rock fall or a large flood. The clasts are angular, meaning that they traveled quickly and didn’t have time to become rounded through abrasion with other grains, as you may observe in river pebbles. Because this is a carbonate breccia, it means that the grains are pieces of carbonate rocks that have been broken apart and transported (in this case, most likely via a rock fall), and have been cemented together with smaller carbonate mud grains. (Mud, in geology, refers to a very small grain size.)
(For scale, the thin beds to the right are centimeter-scale.)
Finally, to end the day, we hiked along this stream bed to look at the rocks that have been exposed by the erosion of the water (which isn’t flowing much today).