This slide focuses on rocks of Cambrian to Late Devonian age (550-350ma). So this period is twice as long and the previous discussed one (200 vs. 100 my). During this period sedimentation continued on the passive margin with the same distribution as before, but in this period there are a number of differences. First, the sediments deposited are not as thick as from the last period (latest Precambrian to earliest Cambrian). Also the sediments are not near-shore non-marine, but are dominantly near-shore, shallow water, marine limestone. The sediments still thicken to the west with the hinge line still near Las Vegas.-------------The reason for many of these differences has to do with the increasing maturity of the passive margin. When the rifted edge of the continent was a relatively young feature, the thin crust and the high heat flow from the underlying mantle made the crust weak. This weak crust was susceptible to flexure as the margin was loaded with early sediment, but during this period the crust has cooled and thickened and grown more rigid. This reduced the subsidence rate and subsequently the amount of accommodation space available for sediments. With limited subsidence, the thickness of the carbonate deposits were limited by sealevel and thus did not become tremendously thick.--------------Speaking of sea level, this period (Cambrian to Late Devonian age (550-350ma)) was also one of large fluctuations in sealevel and we can see in the sediments deposited on and east of the margin that the shoreline fluctuated broadly across western North America. The sediments deposited inland of the margin were thin at takes a great deal of local loading to cause flexure in the thick and old continental crust. ---------------The fine grain distal marine deposits that were deposited off the slope during this time are not preserved in place, but rather were (at a later time) structurally thrust up and over the nearshore marine rocks (this will be discussed shortly). ------------------Also during this period, there were active island arcs off the western edge of North America that were creating volcanic edifices in the ocean. The fossil assemblage of the marine sediments that were deposited adjacent to these volcanics have been found to be the same as those found along the passive margin, demonstrating that the island arcs were not far from the passive margin. These island arcs were later accreted against the western edge of North America, but we will talk about how that happened in a future lecture.
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