This map depicts the latest Precambrian and the earliest Cambrian. The sediments that you see with contoured thickness, thickening to the west are the first sediments deposited on the newly established rifted margin. This is also referred to as a Passive Margin as there is no active tectonic deformation aside from subsidence driven by the weight of the sediment loading and flexing down the warm and weak western edge of the continent. We know roughly when the rifting happened because the first sediments (localities denoted with triangles) deposited onto the passive margin are sediment from the extensive late Precambrian glaciation. We will discuss these particular deposits in detail after a few more slides. ----------------------The deposits on the passive margin thicken to the west, and are largely non-marine, nearshore clastic sediments, reaching thicknesses of up to 8 km! This great thickness reveals a couple of important things. First the sedimentation rates were extremely high so there was a great deal of sediment available to be deposited on the new passive margin edge. But not only was there a high sedimentation rate, but the depositional environments were relatively consistent through time at a locality. This suggests that even though there was a good deal of sediment moving off the continent, the thinned basement rocks off the edge of the continent were subsiding rapidly under the weight of the sediment, keeping the depositional interface at roughly the same elevation (thus the consistent depositional environment). It is the rift faults and the warm weak crust that allow for rapid subsidence following rifting. So as the continent is becoming loaded and thinned through extensional destruction of the continental edge, the continents is undergoing a constructional period as the sediment is accumulated on the margin. An interesting thing about this passive margin is that we can’t see a continuous section leading to the distal, extreme western edge of the margin. ---------------Another thing to note in this image is that we presently see discontinuities in the passive margin sediments. In the south, the passive margin appears to have been offset by left lateral movement on a fault ~250 my ago (late Permian, early Triassic). The continuation of this passive margin is now somewhere in central Mexico. There is a northern location to where the passive margin seems to be offset. This offset happened sometime in the early Jurassic or the early Cretaceous.
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