The document summarizes a field trip to study turbidites in the Ordovician sandstones of the Poughkeepsie area. At two stops, the students observed thin-bedded turbidites consisting of alternating sandstone and shale beds deposited by turbidity currents. They identified sedimentary structures including load casts, flame structures, and ripple marks. By measuring a stratigraphic section and analyzing cross-bedding, they determined the paleocurrent direction was northwest. The outcrops showed evidence of being deposited in a submarine fan environment.

1. Gloria Gill Field Trip Summary In our Field trip to the Ordovician Sandstones of the Poughkeepsie Area we observed and studied thin-bedded “Classical Turbidites.” According to Roger G. Walker, during a short lived event, a turbidity current quickly deposits alternating beds of graded sandstone and shale, that once deposited, they are rarely reworked. Therefore strata and sedimentary features such as load casts, flame structures and ripple marks are well preserved. In general, we observed areas of strata at both stops that fit this description well. The Walker Submarine fan model illustrates that sand stone beds vary in thickness, grain size and sedimentary features depending on their location in the ancient submarine fan when they were deposited. Based on this model, the strata at both sites are indicative of Classic Turbidites that consist of proximal upper fan facies overlying distal lower fan facies. By measuring a stratigraphic section my partner and I were able to identify bouma sequences and draw conclusions about the ancient turbidity and paleocurrents that created these structures. Stop 1 consisted of a large over-turned stratigraphic section, about 20 meters thick. The sandstone beds thicken up as you walked along the road cut. Graded bedding was also visible within the massive sandstone. On the sole of the sandstones there were features such as load casts and flame structures that indicate that the clay was not fully lithified when the sand was deposited on top of it. Rip up clasts are also evidence supporting this claim. Such features are useful to determine which way is up. We observed combinations of parallel laminae, ripple cross laminae and convolute laminae through out the sandstone. Using cross laminae ripple marks we were able to establish that the paleocurrent direction was to the northwest, consistent with what is known about this area during the Ordovician Period before the Taconic Orogeny. Speaking of the Taconic Orogeny, there was a large fault area about half way into this exposed section. Although we continued to record the stratigraphic section beneath the fault as best as possible, we were skeptical about which way is right side up in this area. The stresses and strain that this area endured during the Taconic Orogeny needed to be taken into account before drawn conclusions from this area. Some sections were so obliterated and complicated that they are not any use to my study at all. Although the last 5 meters of the outcrop also may be affected by the fault, they are very interesting and worth mentioning. The first meter consists of ten thin sections of alternating eolean deposits of sandstone and mud. As you progress through the next couple meters, the sandstone thickens upward, indicating progression to lower fan deposits. Unfortunately, we did not have enough time to measure a stratigraphic section at Stop 2, however this stop had an abundance of beautiful ripple marks and flame structures. Even with out a detailed study, evidence for parts of a bouma sequence were apparent. There was massive graded sandstone, followed by parallel laminae, ripple marks and topped off with shale. Loading structures on the sole of the sandstone were plentiful and this was truly a textbook example of flame structures. Some of the sandstones at this site are composed entirely of climbing ripple stratification. The significance of these climbing ripples is that they are indicative of an ancient unidirectional flow with excess sediment supply. I was not able to find an area where paleocurrent analysis could be generated. Another student did take strike and dip directions but fail to give a compass dip direction; therefore I was unable to use that data. I truly wish I had more time to spend on this outcrop; its features were almost ideal. I am grateful that I had to opportunity to study Turbidites in the field. This experience has enabled me to have a deeper understanding of turbidity currents, paleocurrent analysis, bouma sequences and stratigraphy in general. In addition, I learned to take detailed field notes and to map a stratigraphic section, skills that will aid in my future research. Reference: Petro and Schwab, p 189-197. Walker, 1984, Turbidites and associated coarse clastic deposits; in Walker, R.G., Facies Models, 2nd Ed., p.171-188