Question

At the beginning of Paleozoic, was the Cordillera an active or passive margin? In the Ordovician and Silurian, conditions changed and graywackes and volcanic rocks were deposited in a trench to the west and balck shales and cherts were deposited in the back arc basin to the east. Would graptolites be found in the rocks to the west or to the east?

Answer 1

Graptolites would likely be found in the back-arc basin to the east, where black shales and cherts were deposited during the active margin conditions of the Ordovician and Silurian periods.

Step-by-step explanation:

At the beginning of the Paleozoic era, the Cordillera was likely a passive margin, but in the Ordovician and Silurian periods, conditions changed to an active margin as indicated by the deposition of graywackes and volcanic rocks in a trench to the west and black shales and cherts in a back-arc basin to the east. The presence of graptolites, which are marine fossils, would be more typical in the rocks deposited in the back arc basin to the east, where black shales and cherts were deposited.

These environments would have been suitable for the marine life of the time, which includes graptolites. Since graptolites are particularly associated with deep-water shale and chert deposits from the Paleozoic era, we would expect to find them in these specific sedimentary rocks which typically accumulate in quieter, deeper marine settings like the east side's back-arc basin during active tectonic periods.