Lecture Notes

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Earth, Atmospheric, and Planetary Sciences

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Fall 2005

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Undergraduate

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Lectures for Field Geology are presented as slide shows. The image galleries below correspond to lectures 3, 4, 6 and 7. All pictures are accompanied by lengthy captions, summaries of the spoken comments on each picture by Prof. Burchfiel.

Lecture 3 Image Gallery: Field Logistics

Lecture 4 Image Gallery: Passive Margin to Antler Orogeny

Lecture 6 Image Gallery: Antler Orogeny to Permian

Lecture 7 Image Gallery: Permian to Present

Lectures 8-13 were seminar-style paper discussion sessions.

Except as noted, all images courtesy of Professor Burchfiel.

We do our field work in the southwest United States in January. It is true that there is snow in the high mountains in the desert at that time but we will not work that high. We try to work in snow-free regions. There is a lot of walking. As the regions we are mapping are often not accessible by car, we will walk in and out each day so it is important to be in good shape. This is an image of the camp set-up. There is a cook tent in the background. Inside are stoves, tables, food storage and lanterns for the evenings. We wash our own dishes and you can see them drying on the table. There is also a cooler for our veggies and drinks. As we are at a remote location in the desert, water is scarce. The red water cooler is for drinking during the day, but to be conserved when washing. There will be three showers during IAP or about once a week or 10 days. There will be mapping groups that will also have roles in the kitchen. These will alternate day-to-day, but each group will have responsibilities to help in food preparation and in washing kitchen dishes. This is the map tent where we store the computers and the mapping equipment. It is where we will eat if the weather is bad but most of the time we will use this tent for doing the digital mapping work at night. We have generators and batteries for electricity but will use gas lanterns for light. These tents are high quality arctic tents and have served us well. We purchased them with gracious support from the Kaplan Foundation. There is a lot of wind in the desert, especially at night and, our old tents were knocked over frequently. We will often have discussions in the morning that will cover what we are doing each day in the field. Instructors or mapping teams will let the others know how their work is progressing and what to think about during the day. From the low angle of the shadows you can see we start early in the day. Those groups responsible for breakfast preparation start a half hour early. We also make our lunches during breakfast time. This is your chow for the day and is very important to make a good lunch. The saying goes, “a geologist is never far from his/her lunch.” When doing field work it is never a good idea to leave your food or pack behind as you might intend to just run up quickly to one outcrop, but in the end you may end up getting excited and follow a feature half way across the field area before you realize it is time for lunch and your food is way back over there. These are the old tents. Though large they got knocked over frequently in then nightly winds. We have a set of 4x4 field vehicles that we use for moving students between their daily field mapping areas and the camp. Here is another morning camp scene. You can see the coolers circled as benches and peanut butter still out and on the table. The water jug down to the left of the cook tent is how we keep ourselves supplied. You will need to have adequate water bottles for during the day wile you are mapping. Here are our tent-sites spread across the desert at one camp. Tents MUST be strong and wind-proof and well staked down. Dome-style tents that are low to the ground are the best. One year, during the day, a gust of wind pulled one students tent out from under its fly, and tumbled across the desert never to be found. We searched miles down wind and never found it. The sleeping bag was inside as well! Other instructors will often talk about different geologic topics: geomorphology, surface processes, mineralogy, tectonics, structure, geochemistry, etc… These will help guide your observations and work in the field. We will stay together as one big group on the first day. We will introduce you to the regional geology and the rocks that we will be working in. This will be your only introduction to the field area. The rest of the time you will map in independent groups. Instructors will rotate among the groups, but it will be ultimately up to you to figure out what is going on. The best way to learn field geology is just to get out there and do it. This is another image of a field camp. Again note the good strong dome tents. This may be an image from one of the Environmental Field camps. Here you see a party of two mapping together, collecting data and discussing it. In this dark image you can see a group of geologists examining an outcrop…When we go into the field we try to go to an area that has not been mapped in much detail before. Some regions in the west have been mapped over and over again and trails follow the contacts and that takes all the fun out of discovering the geology. Plus, doing work in a new area means making a contribution to our geologic understanding of the southwest. This contributes to science. This also means that the instructors are just as unfamiliar with the field area as the students are and it gives the students and opportunity to observe how a geologist really goes about doing their work. It keeps the instructors curious and on their toes as well. Geology is not always straight-forward and there will be times when we are all confused together and we will have to find the best explanation we can. When the weather gets bad (snowed-out or blow-out), we often go on field trips. This is one to Death Valley. This is another image from that Death Valley side-trip. These trips build on your regional understanding. Here is an aerial view of the tents and camp. Again, you can see the good dome tents in the background. This is why we do work in the desert! Great exposure! In New England there are trees and soils and brush that obscure 80% of the ground. In the desert there is ~100% exposure. Structure and rocks are well exposed and visible to be mapped. For all the hard work we do during the day, there are beautiful evenings that reward us at the end of the day. This image is of the moon at sunset. Some Final Considerations: (Lectured with out slides)----------There are no weather reports where we will be. We deal with the weather we are given.-------We use digital mapping tools to expedite the map-making process. Many maps have been made in the past, but the process of getting those maps to publishable form is to cumbersome, so many maps are never completed for others to see. We hope to overcome that with these digital tools. The map we make will be in publication-ready form at the end of the semester. Using digital tools in the field also allows us to have multiple datasets available to us at the outcrop including Landsat and other remotely sensed data.--------At the end of each day, we will be digitizing our mapping results, adding both our point data and our line data into the GIS. This way, each morning, we can print out a fresh map, summarizing all the other group’s work which helps us find holes and focus areas on the map.------------The course layout: Dr. Burchfield will lecture the first half of the course about the geology of the southwest US. In the latter part of the semester, the lectures will focus on our specific field region. We will work mostly out of the GSA Decade of North American Geology (DNAG) which summarizes the tectonics of the western US.

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!

Unless otherwise noted, all images are courtesy of Professor Burchfiel.

In this lecture we return to the Late Devonian/ Early Mississippian to discuss the Antler Orogeny. In this image we see the sheared and folded deep water sediments of the distal Antler marine basin thrust over and emplaced on top of the shallow water deposits of the continental margin. It is uncertain how and why these deep water sediments were thrust up and over the continental edge extending from southern California to Alaska. We have a good idea that they were not transported much more than a few hundred kilometers as the sediments contain fossils of North American affinity but lack and volcanic or Arc materials. To understand the story of what happened when, we rely on the sediments deposited at the time that the events were occurring. The Antler foredeep (introduced in a previous lecture) records the sediments derived from the mountain building event that occurred as the deep-water sediment overrode the shallow sediments. In the foredeep, we see both sediments derived locally off the topographically high Antler thrust sheet and those from the long traveling continental rivers. The Antler sediments, transported from the west are composed of conglomeratic deposits composed of cherts, shales and other low-grade metamorphic rocks. This is the first time we see sediments derived from the west- a feature that indicates the first tectonic activity and mountain building in the west. Transported from the east, we find finer-grained, quartz-rich sediments that have traveled great distances (during transport, other minerals are eroded away, weathered or deposited, leaving quartz grains preferentially carried over large distances). In this image we see the Robert’s Mountain thrust contact up close. Above the contact we see the Early Missisppian conglomerates (but shouldn’t this be the distal sediments) and below we see a thick band of sheared rocks. To constrain the age of a thrust fault like this, you need to know the age of the youngest rocks in the footwall and the oldest age of the sediments produced by the deformation (the oldest age of the foredeep sediments). For the Roberts Mountain thrust, we know that the fault was active over roughly 10 million years. A possible young analog for the Antler Orogeny. In this image you can see a big thick stratigraphic section on the eastern edge of the foredeep where continental sediments and carbonates continued to do be deposited. Here there is no record of the Antler foredeep or orogeny. If we move west from the previous slide we see the continental margin sediments sourced from the east inter-finger with the conglomerates sourced from the Antler Orogeny in the foredeep. In the central part of the foredeep, the sediments are 5-6 km thick, but if you go east, out of the foredeep, toward the continentally derived sediments, you find for the same time period sediments that are 300-400m thick. In this image we see where early Pennsylvanian sediments were deposited over the eroded interior of the mountains of the Antler Orogeny. This suggests that the topography created by the Antler was eroded to low relief that the Antler Orogeny was complete by the early Pennsylvanian. In this image we see folded carbonates that were deposited west of the Antler Orogeny at the same time that that deformation was occurring. This suggests that there was no major continental or island arc collision that created the Antler Orogeny. In this image we see the ashes and sediments associated with the late Paleozoic island arcs that have experienced epidote alteration. That is what gives them their blue-green color. Though these volcanically derived deposits were created at the same time as the Antler, there are no evidence of ashes or volcanic fragments in the Antler-related sediments, suggesting that the Arcs were still far away from the edge of North America during Antler time. -------------- An interesting thing to know about the sediments created in Arc Complexes is that most of the volume of material that we see is not the primary eruptive deposits but rather the materials eroded off the volcanic edifices. Thus they are mostly sedimentary deposits derived from volcanic materials. In this image we see a student contemplating the rocks further west of the previous image. There you find the ocean floor on which the Arc was deposited. The structurally lowest rocks we find below oceanic crust are tectonically layered dunite (the olivine-rich mantle) and peridotite (olivine-pyroxene). Though these rocks are the source of the rocks that we traditionally think of as igneous, we would call these rocks metamorphic because they are under temperatures and pressures where they are ductilly deformed. In this image, the bands that we see in this rock are due to ductile flow in the mantle adjacent to the mid-oceanic ridge. In this image we see the layering of crystals that accumulate and precipitate out of the melt in a magma chamber. There are light colored trondhjemites and mafic peridotite but most of the cumulate in the chamber is a coarse gabbro. So in this image the layering is not by tectonics or metamorphism but rather my crystals setting out of the melt through time. The mafic minerals precipitate first, then sequentially other minerals rain out of the melt and accumulate on the floor of the magma chamber. Banding or repeated crystal precipitation events suggests that the chemistry of the melt is changing in time as new magma enters the chamber or as the melt slowly cools. Though there can be variations due to the influx of new magma, the general progressive trend toward cumulates that are more feldspar rich provides geologists with good right-way-up indicators, an important tool for geologists mapping in field areas with extensive deformation. As you move up through the oceanic crust, you leave the magma chamber and enter the sheeted dike complex. This region is composed of a collection of vertically-oriented basalts in dikes, but there are so many that this region is almost entirely composed of dikes. In this image you can see one of these darker dikes cutting up and left though a basaltic rock that is just composed of dikes itself. The darker rocks are along the dike margins due to rapid cooling against lighter-colored rocks near the middle of a previously intruded dike. The extension responsible for creating the fractures that the dikes travel up is due to the tectonic instability of the region adjacent to the mid-ocean ridge. In this image, we see the part of the oceanic crust just above the sheet dikes complex: The Pillow Basalts. The production of rocks is fed from below by the dikes emerge into the deep sea and erupt on the surface of the seafloor. The bulbous form that we traditionally see is due to the eruption of magma fed from the dikes below, but rapidly cooled on the sea floor. The forms are rounded on top and pinched at the base as they fill the convex space between those basalts already erupted. In the cordillera we can see portions of the Cambrian ocean floor. In this image we see folded and deformed cherts that were deposited above the pillow basalts. This picture is from the area west of the Roberts Mountain Thrust in the Klamath Mountains of northern California. The typical stratigraphy of the ocean floor has to so with the changing depth of the seafloor as oceanic crust is created at the spreading ridge and moves away. Near the spreading ridge, the crust is warm and buoyant and at shallow enough depths that carbonate deposits are preserved. As the crust cools and moves away from the spreading center it is less buoyant and is found at deeper depths. As the depth increases, the temperature of the bottom waters decrease. The sediments found in these deep-water regions are mainly silicious deposits, such as chert, shown above. This is because carbonate’s solubility increases with decreasing temperatures. Though throughout the ocean, carbonate detritus created by marine microorganisms rains out of the water column, the deep parts of the ocean (which are cooler) lack carbonate sediment because the falling detritus dissolves before it can be deposited. The depth at which this happens is called the CCD or the Calcite Compensation Depth. Thus the stratigraphy of ocean floor sediments typically starts out as carbonates, but later become silicious. There sediments complete the makeup of oceanic crust. If we find all these components present on land, then we can call it an Ophiolite Sequence. Lets review. Peridotites and Dunites with metamorphic ductile fabrics are at the contact between the mantle and the crust. Above there are magna chambers with cumulate textures due to the progressive settling of crystals from the melt. Fed from the magma chamber, the sheeted dike complex is typically composed of many, many generations of near-vertical dikes. These dikes reach the surface and provide the magma erupted at the ocean’s floor as pillow basalts. Pelagic sediments form the top of the oceanic crustal sequence. In this image, we see some of those arc volcanics deposited above seafloor sediments. This next image portrays the timing of different depositional events around the time of the Antler Orogeny. You can see that the Arc Volcanism continued throughout the time that the Antler was created. You can also see that the sediment found in the Antler Allochthon were experiencing active deposition until ~360ma. The filling of the foredeep then starts ~355ma, suggesting that the material had been thrust over the sinking continental margin. By ~350ma, we find sediments overlapping the Antler Orogeny, suggesting that it had been significantly eroded. The dates in this figure are old, and though we do not need to know the exact numbers right now, it is important to think about the different forms of geologic evidence available for constraining when certain events happened. Note that deposition continues in the Havallah basin east of the arc and west of the developing Antler Mountain belt. This suggests there was no arc collision during Antler time. Three additional events are important during Mississippian-Permian time in western North America: the formation of the ancestral Rockies east of the Cordilleran belt, the formation of the Havallah Basin and the strike-slip formation of the southwest continental margin. This latter event set the stage for Mesozoic subduction beneath the continental crust in the southwest Cordillera. The Havallah sediments, deposited in a distal marine basin, were a lot like the Antler sediments before they were thrust over the western edge of North America. In the Permian, these marine sediments were thrust up and over the western edge of North America, overlapping the rocks of the Antler Orogeny. No oceanic crust in found in either the rock of the Antler or the Havallah Orogenies. The sediments could have been scraped off a down-going plate like in Accretionary wedges. The difference between the two events is that the accretion of the Havallah basin sediments was driven by an Island Arc collision. The island arc emplaced west of the Havallah sediments had an accretionary wedge on its western edge that contains marine sediments that range in age between Cambrian and Permian. These accretionary wedge sediments contain a fossil assemblage that is not of North American but rather of Asian affinity. This fossil evidence suggests that during arc formation vast areas of oceanic crust were subducted beneath the west side of the arc, some coming from the Asiatic side of the PaleoPacific. We know that the Havallah basin was close to North America as its fossils are of North American affinity. Some workers debate whether the Havallah basin did not directly collide with North America, but rather was transported laterally into position along large transverse faults like those found on the South Island of New Zealand today. An onlap sequence of sediments of Triassic age cover the Havallah and Antler rocks, demonstrating that the mountains created by these accretionary events were eroded away and subdued enough to have shallow marine rocks deposited over them. In this slide we see the igneous rocks that intruded the edge of North America exposed by the late Paleozoic truncation fault. These granites are ~252ma and are overlain along an unconformable contact by conglomerates composed of the same granitic materials. This granite suggests that the edge of south-western North America were subject to subduction following truncation and formed the beginnings of a Continental Andean arc sometime before 252ma.

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