Except as noted, all images are courtesy of Professor Burchfiel.
Geologic map of North America. On the west side of North America, adjacent to the Pacific Ocean, is the Cordilleran Mountain Chain, the main focus of our regional geological discussions.
This image depicts the two major tectonic provinces in western North America. On the west are accreted terranes of Paleozoic to Cenozoic age. Each terrane is composed of fragments of oceanic or continental material that were created somewhere away from North America, but were then tectonically accreted or attached to the western edge during subduction or translation along strike slip faults. This material built out the western edge by accreting island arcs or continental fragments translated here from far away (exotic) or close by North America (indigenous). ------------------------ On the Eastern half are the older passive margin sediments that were largely eroded from the eastern old part of North America and transported westward and deposited at the western edge of North America.
This image shows that the accretionary part of north America is actually composed of a large number of individual terranes. We will go into their origin and translation to their current position in future lectures.
This image shows the distribution of Precambrian rocks in North America (NA). This means that the rocks are 600 million years old (ma) or more. In the NA Precambrian there are some blocks that are older than 2.5 billion years (by) old. You can also see in this map that there are younger Precambrian strips snaking between the larger blocks. These strips represent the mountain building events that happened as these older continental fragments slammed against one-another. They are now eroded down to a low relief surface, but can be recognized as the suturing together of the larger older continental (aka cratonic) blocks. In the south part of NA you can also see that there are some younger blocks that were added onto NA, and in these blocks you can see interesting places where parts of the blocks are missing. In particular, the western edge of NA seems to have been removed from many of the western Precambrian blocks. In this course we will emphasize how mountains are created through the process of first rifting (or tearing) apart of continents and creating ocean basins, and then fragments closing those basins as continents collide with one another. When we look at the age of the ocean floor across the globe, we see that the oldest oceanic crust that we can find is not much older than 200 million years. But in the continents, we see rocks that are up to 4 billion years old! That is because oceans are transient features that are born and destroyed and they are subducted beneath continents or accreted against their edges. This is a great thing, because that means that the earth’s history is recorded in the continents, not in the oceans…if it were the other way around, it would be tough, wet work doing geology. So, we can see from the way that the Precambrian continent is broken off on the western edge that there was rifting and the development of the passive margin sometime younger than 1 billion years.
This map shows the actual outcrop distribution of Precambrian rocks in the Cordilleran region. This way you can see where the data comes from that we used to make the previous slide. In the end, few Precambrian rocks are exposed at the surface. The ones we are going to focus on are the blue ones next to the western edge of Precambrian rocks. These rock are some of the first sediments that were deposited along the western edge of the continent following rifting. They are 600-650 m.y. old and are not continuously outcropping along all of western NA, but can be interpreted to be the first sediments deposited near the passive margin. We will discuss these more with some slides to come. In the north here you can see a huge outcrop of older rocks, the Belt Group. They are 1.5 by to \<1 b.y. old. The Belt rocks are still a big puzzle because (1) the western half are heavily metamorphosed and difficult to interpret and (2) they are also a super-thick pile of sediment, up to 15 kilometers! How is this possible? We know that the only way that this was possible was that the crust was extremely thin so that the sediments could be deposited. The Belt group could have been deposited in a weak thin part of the crust that was associated with the rifting along the western edge of NA before the passive margin development at a later (~600-680 m.y.) time. Still, the Belt Group remains a big mystery.
Here is an image of the unmetamorphosed portion of the Belt group. As you can see, they are a thick pile of sediments!
This map depicts the latest Precambrian and the earliest Cambrian time. 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 driven by interactions between plates. In the passive margin setting, the only deformation is extensional and is a consequence of sediment-loading causing subsidence of 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 and 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 and there was a great deal of sediment available from the continent to the east 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, keeping the depositional interface at roughly the same elevation (thus the consistent depositional environment). It is the extensional rifting 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 are 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 section continuous across strike leading to the distal, extreme western edge of the margin.
In this image you see folded shallow marine sediments. This is from the first phase when the rate of subsidence was matched by the rate of sedimentation.
In this image, you can see the typical near shore or river environment where the sediments have well defined cross bedding and there are desiccation or mud cracks in the rock, suggesting subaerial exposure.
In this image you can see a stromatolitic limestone that would represent an algal mat community that probably existed a little ways offshore.
In the lowest part of the passive margin sediments you see rocks like this: coarse, poorly sorted, angular to well rounded sedimentary clasts supported by a fine grain matrix. Sedimentary rocks with this texture are know as a ‘Diamictite.’ ------------------- The interesting thing about this diamictite is that some of the clasts have scratches or shallow linear groves cut into them. These groves are indicative of glacial transport. When the glacier is sliding down slope on the basal bedrock, there are often clasts that are imbedded in the ice that are dragging across the underlying rock. This dragging creates the groves or ‘striations’ on the clasts. -------------------- There are no dateable ash horizons or other opportunities for direct dating. But, if the texture and striations demonstrate that this deposit is indeed glacial, then we have a good sense of the age of the initiation of sedimentation on the passive margin of North America. If these deposits belong to the well-studied, globally synchronous glacial period in the late Proterozoic, then we can bracket the initiation of sedimentation somewhere between 650 and 600 ma. (this image is from Canada)
This image is of the equivalent basal diamictite but in southern California…suggesting that the glaciation was laterally extensive (if not continuous) across western North America.
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 and subsidence during cooling as the margin was loaded with early sediment, but during this period the crust has cooled more slowly 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 but at times reach as far east as Wisconsin. Volcanic arcs began to develop within the PaleoPacific Ocean far from the North American continent. ---------------The fine grain distal marine deposits that were deposited off the continental rise 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).
This image is a cross section demonstrating the two phases of deposition on the passive margin of NA. The lower part of the section is the first phase of deposition and you see it was dominantly a thick sequence of non-marine nearshore deltaic and sandy deposits. The second phase was after the strengthening of the margin, the slowing of subsidence and the deposition of nearshore, marine carbonate deposits. You can see on the left side of the slide that during the second phase we also had the deposition of the distal, fine-grained materials that had been thrust back over the passive margin. Again, Las Vegas is at the hinge line, and you can see that there was discontinuous deposition east of there, it just was not as thick as on the subsiding passive margin. This slide also raises the point that we must be careful when we interpret cross sections, at the vertical exaggeration can be deceiving. You can see below the unexaggerated section.
Here is an image from the Grand Canyon. Remember, we are east of the hinge line. In the image we see 1.6 – 1.8 by basement rocks overlain by non-marine and marine rocks that span the early Cambrian to the Devonian…but here the section is only several hundred meters thick. Younger Mississippian rocks make up the upper cliffs.
This image is from Death Valley, well west of the hinge line. The pile of Cambrian sediments here are ~3000 meters of carbonate rocks are represent only a small portion of the rocks that represent the same time period we just looked at in the Grand Canyon. While in the east the sequence is ~200 meters thick, the same sequence in the west is greater than up to 10,000 meters thick! This is what subsidence can do!
This slide shows those distal rocks that were deposited during phase two, but thrust back over the passive margin during the Antler Orogeny (mountain building episode). It is hard to say exactly how thick the deposits were or what the sedimentation rate because during orogeny the rocks were highly deformed and imbricated like cards to that there is a great deal of repeated section.
These are the highly altered volcanic rocks of an island arc that have been accreted against North America at the end of the Paleozoic. The rocks are green because of chlorite alteration during the circulation hydrothermal waters near the volcano. The rocks are Cambrian, the same age as the slates and distal rocks thrust up during the Antler Orogeny…but these are tough to work in…they are a geochemist’s nightmare. Hats off to those who can do this work!
And this slide is in here just to show that though I may present a relatively coherent story of the western edge of America, there are still things that we do not understand. These rocks are a laterally extensive Paleozoic quartzite and we just dont understand where they were deposited. All the deformation and accretion leave the rock record in a state where deciphering what happened when is never easy.
The final part that we will discuss today has to do with the End Devonian to the Early Mississippian (~350-340 ma). During this period we find the western edge of NA switching from a passive margin to an active one where deformation due to subduction has thrust the distal sediments of a ‘phase 2’ age up and over the near-shore passive margin sediments. This mountain building event, called the Antler Orogeny and was driven by convergent mountain building whose origin remains a subject of much controversy. This constructed a large mountain range bounded on the east by the leading edge of the thrust (the Robert’s Mountain thrust). The loading of the crust due to the emplacement of the thrust sheet created a flexural depression in the crust (the Antler Foredeep) on the east side of the mountains. This depression or foredeep was then the locus of deposition for sediments eroded off the Antler Orogeny. The age of the distal sediments overthrust in the Antler Orogeny are the same as the near shore passive margin and the fossils within the distal sediment are also of North American affinity. This then explains that this overthurst material was ‘indigenous’ (or contiguous with the footwall rock) rather than ‘exotic’ as the island arcs are. Though the Antler Orogeny was due to the interaction of convergent activity along the western edge of north America, it would not be the last. There are plenty more to come!
