History of the Earth

Columbia River Basalts (yellow) - see below for source. During the Miocene epoch of the Cenozoic, about 16 or 17 million years ago, the Pacific Northwest of the United States was a busy place.  A hotspot, a relatively small location where heat is focused upward from deep in the earth’s mantle, either reached shallow depths, or North America in its movement westward encountered one. At about what is now the common corner of Oregon, Idaho, and Nevada, the hotspot’s heat brought out lava – lots and lots of lava. The Columbia River Flood Basalts are comparable to those in Siberia, and the Deccan in India, and the Parana Basalts of South America. Over about two or three million years, 17 to 14 million years ago, something like 40,000 cubic miles of basalt was erupted, mostly in what is now Washington and Oregon. There are at least 300 individual flows stacked upon each other. In area and volume, the Columbia River basalts are tiny compared to the Siberian flows, and about one-third the size of the Deccan, but still pretty large, and they are among the youngest of these flood basalts.  It looks like there was a north-northwest trending zone of weakness that extended away from the center of the hotspot – or maybe the hotspot was asymmetrical, or bigger than usual – so that the cracks through which the lavas came were focused to the northwest in Oregon and Washington. Another big crack extended to the south-southeast, through northern Nevada, producing the Northern Nevada Rift, a narrow zone of igneous rocks of Miocene age. Flood basalts didn’t flow there, though, perhaps because the region was a little stronger, a little more a part of the North American craton than the country in Washington and Oregon. Hotspot origin of various features (see below for source)At the site of the hotspot itself, that corner where Oregon, Idaho, and Nevada come together, huge explosive volcanism took place. While the flood basalts came out relatively quiescently, like the flows in Hawaii today, the center was a scene of violent activity. A caldera developed. This is a huge collapse feature that forms when a magma chamber erupts much of its lava, leaving a void behind. That empty space may collapse, with the surface rocks falling down into the old magma chamber. The first caldera related to this hotspot, near that corner of Oregon, Idaho, and Nevada, is about 35 miles across. As North America continued to move southwestward, the position of the hotspot was progressively further and further to the northeast. Today, it is under Yellowstone National Park – the Yellowstone Hotspot.  The trace of North America’s movement over the hotspot is clearly defined by a series of calderas that get younger and younger as you go from the southwest corner of Idaho to Yellowstone. They are in the Snake River Plain of southern Idaho, which is covered by basaltic and other volcanics associated with the various calderas. Ages of Yellowstone Hotspot Calderas (illustration by Kelvin Case at English Wikipedia, used under Creative Commons license) Our other discussions of extensive volcanic events have often found some correlation between the volcanism and extinctions. Was there one with this one? About 14.5 million years ago, about 2 million years after the flood basalts started and while they were still in progress, there was a marked global cooling event that coincided with a major growth spurt in the Antarctic Ice Sheet. And it does correlate with an increase in extinction rates, though I don’t think we’d call it any kind of mass extinction like the great ones in earth’s history. This may have been mostly a result of the change from what’s called the Miocene climatic optimum, a warm period 17 to 15 million years ago, and part of a more general change to cooler conditions that eventually led to the ice ages. It’s not obvious that the Columbia River Basalts played a major role in this minor extinction event at 14 million years ago. We’ll talk a bit more about Y

The Caucasus Mountains, between the Black and Caspian Seas, hold one of the most important and early-produced oil provinces in the world. This area is part of the Alpine-Himalaya collision between pieces of Gondwana and the southern margin of Eurasia. Specifically, it’s the northern prong of Arabia that’s squeezing a small bit of continent, more or less part of the main Iran block, which itself was part of the Cimmeride continent, all that is being pushed into the south side of Eurasia. Geographically, the Caucasus is taken as the boundary between Europe and Asia, and it contains some high mountain peaks, including Mt. Elbrus, a dormant volcano that reaches more than 5,600 meters above sea level, more than 18,500 feet. It last erupted about 2,000 years ago, showing that this area is still tectonically active. Photo: Baku oil wells, Asbrink Collection.One of the effects of the ongoing Alpine-Himalayan collisions was the development of fold belts along and within the Caucasus Mountains complex. Rocks of Miocene age were pushed into large asymmetrical folds, anticlines and synclines with strata arched upward and downward, respectively. This shows certainly that the tectonic action was going on after the Miocene rocks were laid down, since they are involved in the folding. This isn’t a surprise, since we know the collision is still going on today. The early Miocene rocks were probably folded in Miocene time, 5 to 20 million years ago, and in the Pliocene, 2 to 5 million years ago. These anticlines trap lots and lots of oil. Oil was known in the area around Baku from the time of Marco Polo, and was supposedly used by locals for lubricants and fuel in the time of Alexander the Great. Baku oil was produced in quantity from hand-dug wells in the 1830s, and the world’s first paraffin factory began there in 1823. The first mechanically-drilled well in the world was drilled at Baku in 1846, 13 years before America’s first oil well in Pennsylvania, in 1859. By the 1870s, oil demand was surging worldwide, and outside investors came in to develop the oil fields around Baku. Two of the many fortunes that came from Baku oil were those of the Nobels, of Nobel Prize fame, and the Rothschilds. In 1900, half the world’s oil was coming from Baku, much of it from rocks of Miocene and Pliocene age. Further west along the northern front of the Caucasus Range, additional fields were discovered. Grozny, in Chechnya, became Russia’s #2 source of oil until after the Revolution in 1917, and the Grozny area still produced about 7% of the Soviet Union’s oil as late as 1971. The Grozny field is in an anticline in Miocene rocks, with multiple sandstone reservoirs with impermeable shale seals. The Caucasus oil was a major target of Hitler’s forces in World War II, and it still plays a significant role in the geopolitics of the region. Pliocene deltas (that form oil reservoirs) coming into the South Caspian Basin. From USGS Bulletin 2201-IIt’s no surprise that this oil was found so early, because it is practically at the surface in many cases, or just a few feet beneath the surface in the relatively young Miocene and Pliocene rocks. Marco Polo reportedly saw a natural gusher of oil. The organic rich source rocks are largely of Miocene age, called the Maykop Suite. There was a restricted seaway extending through this region, on the north side of the approaching continental blocks before they collided to raise up the Caucasus, and the marine carbonates of the Maykop Suite were deposited there. By Pliocene time, just four or five million years ago, the region became isolated from the sea, and rivers brought sandy sediment into the basin. Some of the most productive reservoirs around Baku are from Pliocene rocks deposited in deltas around the margins of the South Caspian Basin, which is an entrapped bit of old Tethys Ocean floor. The ongoing tectonic activity has created plenty of traps for the oil.  Azerbaijan, where Baku is located, still produces about 900

Today we’re going to a geological province that is almost unique in its nature – at least in terms of its size. The Basin and Range in Nevada and Utah is a region of broken, extended crust nearly 450 miles wide and even longer in a north-south direction. Areas of basin and range topography extend north into Oregon and Idaho, into southeastern California, southern Arizona and New Mexico, and cover a broad swath or northern Mexico.  Shaded relief map (NPS) The name basin and range is pretty descriptive. There are alternating narrow, high mountain uplifts separated from each other by long narrow valleys, or basins. In the core area of the Basin and Range Province, east-central Nevada and west-central Utah, there are dozens of mountain ranges and intervening valleys – 50, 60 or more. The topographic map of the region led one early geologist, Clarence Dutton, to compare the basin and range to an "army of caterpillars marching toward Mexico" – and that’s really not a bad way of thinking of it.   The alternating uplifts and basins, technically called horsts and grabens, are the result of extension of the earth’s crust over this wide area. Take something brittle – continental crust – and pull it from the two sides, and it will break. The breaks are mostly steep normal faults – sometimes more than one – that separate the basins from the ranges. As with any mountain uplift, as soon as there is a variation in mountain relief, erosion starts, and the eroded material was shed into the adjacent basins. In some places, there is more than 10,000 feet of sediment filling the basins, all eroded from the adjacent mountains, which may stand 6,000 feet or more above the valleys. I’ve actually done quite a lot of work on this region because my specialty, gravity and magnetic data, is useful in figuring things out here. The sediments in the valleys are typically much less dense than the rocks in the ranges, so that density contrast is easy to see in gravity data – the denser stuff has a stronger gravitational pull than the less dense stuff. This extension started in the Early Cenozoic or maybe even in very late Cretaceous time. It’s not as if these breaks all just happened suddenly – faulting, while it may generate catastrophic earthquakes, typically only offsets rocks by a few centimeters at a time – or a few meters in really huge quakes. That motion over millions of years can add up to a lot. The early phases of extension in Nevada produced low areas along low hills – nothing like today’s ranges. But the beginning basins were low enough for sediment and even lakes to form. For sure by Eocene time there were at least a few lakes in the region. It’s the Oligocene when the action starts to pick up, with ranges and basins starting to have higher relief, and more movement on faults. There was enough breaking to allow for some pretty vast volcanic activity as well – much of the region today is covered by sheets of volcanic ash falls and ash flows. Most of the volcanism is older than the most recent phase of mountain uplift, because the volcanics are cut by the faults that form the boundaries between basins and ranges, but there has been some volcanic activity in Nevada as recently as the past 5 million years or so. OK, so stretching broke the crust into these long, narrow basins and ranges. What caused the stretching? This is a really big question, and we really don’t have a definitive answer. As with many complex processes, it’s likely to be a combination of diverse origins. One thought has been that the continent-scale uplift of the Rocky Mountains, centered to the east of the Basin and Range, was enough for gravity to drag the western slope of the mountains down to the west, like a gargantuan landslide, and the crust broke as it slid. But the details of the faults show that many of them are not simple straight line breaks dipping steeply into the earth. They are like that near the surface, but then they often curve at depth, merging into a

We’ve been talking about the dismemberment of Pangaea and its biggest piece, Gondwana, for months now. The process is still going on, and the newest break within the old Gondwana continent is in its largest surviving portion, Africa.  Map from Digital Tectonic Activity Map of the Earth (NASA) with annotations by Gibson.The East African Rift System is a present-day break that extends from the Dead Sea in Israel and Jordan, south through the Red Sea, separating Arabia from Africa, and into the African continent through Ethiopia, Kenya and Uganda, eastern Congo and Zambia, and into Mozambique and on offshore. All told, the system is more than 5,000 miles, 8,000 kilometers, long. It’s a big, complex break in the continental crust.  As much as we have a pretty good handle on how rifting proceeds, our understanding of how and why such rifts begin is still pretty poor. There are multiple ideas for how the East African Rift started, ranging from some deep-seated mantle plumes, whose upwelling heat broke the crust apart, to crustal thickness variations that allowed magma to flow upward in some locations preferentially to others, initiating the rift process. Differences in crustal density might have the same effect as thickness variations. Whatever started the rift, it has since followed a pretty standard and expectable development. Early in the process, around 30 million years ago, early Oligocene time, the upwelling magma breached the surface and flowed as extensive flood basalts in what are now Ethiopia, Somalia, Yemen, and adjacent areas. This point is called the Afar Triple Junction, because it is the focus for three branching rifts. To the northwest it’s the Red Sea, a young ocean basin where sea-floor spreading has just barely begun, to the northeast is the Gulf of Aden, true oceanic crust, and the mid-ocean ridge there continues into the Indian Ocean as the Carlsberg Ridge, the divide between the Indian tectonic plate and the African Plate. The third branch of the system extends from the Afar Triple Junction into the African Continent.  Where the rift is in continental crust, in East Africa, the result is long narrow down-dropped troughs, called grabens. They are bounded by normal faults that have large offsets, many thousands of feet in some cases. The situation is very much like eastern North America must have been back in the Triassic as the Atlantic Ocean began to open. In Africa, it’s not one simple linear zone, but it curves and branches into two major segments on either side of Lake Victoria. The fault-bounded troughs, the grabens, are obviously lower that the uplifted flanks, which tend to be mountainous, and the grabens or basins accumulate thick piles of sediment eroded off the mountains. In East Africa, the long, narrow lakes, such as Abaya in Ethiopia, Turkana in Kenya, Lakes Albert, Edward, and Kivu along the eastern border of Congo, Lake Tanganyika between Congo and Tanzania, Lake Rukwa, and Lake Malawi all lie in the down-faulted basins of the East African Rift. Lake Victoria, the second largest freshwater lake in the world, after Lake Superior, isn’t in a narrow fault basin, but it is related to the tectonic activity. It lies between the two big branches of the rift system, and formed when the uplifts to east or west dammed rivers flowing into the central basin. Victoria is a young lake, only about half a million years old or less, and it has dried up completely at least three times in its history, a reflection of changing climate conditions during the recent ice ages. Victoria is a shallow lake, less than 300 feet deep. In contrast, the deep troughs of the rift system hold some of the deepest lakes in the world. Lake Tanganyika, for example, reaches a maximum depth of more than 4,800 feet, and holds about 18% of all the fresh water on earth. Volcanic activity continues in the region related to the rift process, including Mt. Kenya and Mt. Kilimanjaro, and the active volcanoes of Ethiopia and the

First today, I think I have been remiss in not pointing you to a great paleontology podcast, Palaeocast. All things fossils. The most recent episode is about ceratopsians, the group of dinosaurs that included Triceratops. Check it out. And speaking of ceratopsians, the oldest one known was just announced last week. It was a raven-sized little thing that lived in Montana about 105 million years ago. LINK • Link2 * * * Yesterday I promised that today we’d talk about some of the Oligocene debris that helped bury the Rocky Mountains. We’ll go to South Dakota to do that.  You can find the sediments that were shed off the mountains up in the mountains, but because of the Miocene and later erosion, a lot of that stuff is gone, eroded and washed onto the plains or down the Missouri River. In South Dakota’s Badlands, a nice pile of the Oligocene sediment has been preserved. It’s eroded, to be sure, into fanciful angular shapes – that’s why the place is called Badlands, the word for a region of exposed rock, often fairly soft rock, that has eroded into steep slopes separated by an intricate meshwork of canyons and ravines. They tend to exist in arid country today, where erosion rates are relatively low and the erosion that does happen is usually catastrophic, as in flash floods. The rocks eroded tend to be relatively young, because younger rocks are often more poorly consolidated than older rocks, but there’s nothing sacred about that.  Exposure of Oligocene Brule Formation in South Dakota Badlands (National Park Service photo) The White River Badlands of South Dakota preserve a thick wedge of the Oligocene sediments that eroded off the nearby Black Hills, a Laramide uplift. The sediment was carried to the east by rivers of various sizes, some that flowed strongly enough to carry gravels, and some that were parts of shallow braided streams carrying sand and silt. Flood plains saw extensive mud deposits. Over time, from the late Eocene into the early Oligocene, the deposits built up a pile that totaled more than 100 meters thick – 300 feet – in the Badlands National Park area. Across the eastern Rockies region, equivalents of these Oligocene deposits reach thicknesses of 700 feet or more. The first package of rocks that makes up the South Dakota Badlands is called the Chadron Formation, gravels, sands, silts, muds. As the sediment was being laid down, this area was certainly not badland, but harbored abundant life. The Chadron is famous for its titanotheres or brontotheres. The nature of the cement and porosity in the Chadron formation is such that the rocks weather into smooth, rounded hillocks. River and flood deposition wasn’t continuous over the 11 million years of the Oligocene, and in the Badlands of South Dakota, a lot of the reddish layers interbedded with the white rocks are paleosols – ancient soil horizons, red because iron was concentrated in them. There were also occasional lakes in which limestones formed, and one of them marks the break between the Chadron Formation and the overlying, younger Brule Formation. It’s really quite similar to the Chadron, but the cement in it includes considerable volcanic ash and calcite, so it holds together well and tends to form steep slopes and knife-edge ridges. The Brule is famous for turtle fossils and oreodonts. Oreodonts had teeth that were suited for eating oreo cookies – well, no, not really – oreodonts were fox-sized mammals that are not very closely related to any modern animals, but camels and pigs are not too far from oreodonts on the evolutionary tree. Technically they are called artiodactyls, even-toed herbivores. Lots of their fossils have been found in the Badlands rocks. There’s a distinctive volcanic ash bed at the top of the Brule Formation, erupted about 30 million years ago, and it was followed by more deposition in the mid- to late Oligocene of the Arikaree formation, generally the highest unit in the Badlands National Park area. Try to envision a vast sheet

During the Oligocene epoch, which lasted from 34 to 23 million years ago, the Rocky Mountains disappeared. Well, not really, but they were so eroded that to a great extent, they were buried in their own debris. The huge uplifts of deep crystalline rock that were forced upward by the Laramide Orogeny, starting in the late Cretaceous and continuing into the Paleocene, say from about 75 to 55 million years ago, of course began to be eroded as soon as they were above sea level. Because they were well within the continent, rather than on the margins like so many collisional mountain ranges are, the sediment that was eroded pretty much had to go into the basins between the mountain ranges and onto the mountain flanks.  We’ve talked about some of that early Cenozoic sediment, in the Fort Union, Green River, and Wasatch Formations, mostly of Paleocene and early Eocene age. Erosion of course continued – that’s the way it works – so by Oligocene time, about 30 million years ago, the erosional debris, deposited on the flanks of the mountain ranges, had to a large extent buried the mountains.  The sedimentary deposits on mountain flanks are called pediments – from words meaning foot of the mountain – and the pediments extended well up onto the flanks. In some places the erosional debris is more than 3000 meters thick, almost 10,000 feet, and it was deposited to elevations, in present-day topography, as high as 6,000 or 7,000 feet above sea level. So yes, there were still mountains, but not like the high rugged ranges we know today, in the Wind Rivers, Uinta, Big Horns, and others. It would have been more like a relatively smooth, sloping plain, with occasional high crags sticking out of it. That’s how things would probably still be today, but in mid- to late Miocene time, about 5 to 10 million years ago, the whole region began to be actively eroded again. We call it the exhumation of the Rockies, which had been buried in their own debris. What happened to allow that? There are two likely possibilities, both of which may have happened at least partially simultaneously. First, the whole region might have been uplifted. Higher relief allows streams to erode more aggressively, removing things like the sediment in the pediment around the mountains. And second, if the climate changed from arid to wetter, there would have been more water to do its thing and erode stuff away. It’s likely that both happened.  The gentle, regional uplift must have been related to some kind of tectonic activity, and it was at about this time, the Miocene, that western North America was beginning a different type of interaction with the oceanic plates to the west. Instead of simple subduction, the continent overrode the spreading ridge and the contact changed to one with plates sliding beside each other – the San Andreas Fault began to form in the Oligocene, about 30 million years ago, but much of it only became significant in the past 5 to 10 million years. And things were happening to the whole region too – it was pulling apart, forming the basin and range in Nevada and Utah. We’ll talk more about that in a few days, but for now, let’s assume that all this tectonic activity might be a factor in the regional uplift that allowed the erosion to strip the sedimentary cover off the Rockies, carving them into the craggy forms we see today. The recent glacial period put the final touches on that sculpturing, but the stage was set by the exhumation that began about 5 to 10 million years ago, and continues to this day. We can figure this kind of thing out by looking at the sediments themselves – the grains in the rocks can tell us what they were derived from. Was it recycled Cretaceous sand? Or Mississippian limestone? Or was the erosion deep enough that even the Precambrian cores of the Laramide mountain uplifts were being eroded? The nature of the Oligocene sediments tells us a lot about the uplift and erosional history of the Rocky Mountains. Tomorrow, we

First today, for those of you who are into vertebrate fossils, I came across a nice blog report on this year’s meeting of the Society of Vertebrate Paleontologists in Berlin. The blog has a nice summary of new findings, from Archaeopteryx to dinosaur colors, to giant kangaroos. LINK * * * Today we’re going to Antarctica.  Australia and Antarctica began to rift away from each other back in the Cretaceous, probably about 95 million years ago, with the initial faulting and rifting beginning in the Jurassic. The separation was pretty slow for many millions of years, so that by Eocene time the distance between Antarctica and Australia was only about 500 km, or 300 miles. The speed of separation and amount of sea-floor spreading increased in Eocene time from maybe 4 cm per year to 7 cm, and today there’s an ocean basin 2100 miles wide between them, and it’s still growing.  Within Antarctica, all was not quiet. The West Antarctica Rift was the beginning of a pull-apart between the larger East Antarctic craton and the smaller West Antarctica area. It started probably during the Cretaceous and continues to this day. The flank of the rift is more or less the Trans-Antarctic Mountains today, and the active volcanism there including Mt. Erebus is probably related to ongoing activity along the rift. Climate change together with the uplift of the mountains in central Antarctica led to the development of snow cover there beginning about 34 million years ago. The boundary between the Eocene and Oligocene, 34 million years ago, is marked by a decrease in carbon dioxide levels, and general global cooling, which allowed for the beginning of ice buildup in Antarctica. Before about 34 million years ago, there was no ice in Antarctica even though it was near the south pole. The last connection between Antarctica and any other continent, South America, also disappeared about 35 to 40 million years ago. With the opening of the Drake Passage between Antarctica and South America, the circumpolar ocean current was established, and its presence prevented significant interchange of tropical and polar waters. This was another factor in the cooling of Antarctica. The West Antarctic Rift is positioned about where Australia and New Zealand were attached to Antarctica in the old Gondwana. The Tasman Sea, between Australia and New Zealand, began to extend and separate those areas back in the Cretaceous, about the same time as the West Antarctic Rift began. Even though it’s been active for close to 100 million years, the West Antarctic Rift hasn’t really gone anywhere. The active volcanism shows that there is still extension of the crust, breaking, to let the magma up, but an ocean has not formed between West and East Antarctica. The Tasman Sea spread for about 22 million years, 80 to 58 million years ago, Cretaceous into the Paleocene epoch of the Cenozoic, and then apparently stopped. Today, Australia, the Tasman Sea, and about half of New Zealand – the part west of the Alpine Fault – are together on the same tectonic plate. The whole package is pushing against the Pacific Ocean Plate, making for some complex, violent interactions along oceanic trenches north of New Zealand and in collision zones in New Guinea. The oceanic part of the Australian Plate west of Australia itself is subducting beneath the southern part of Asia, in Indonesia. So – the bottom line for Antarctica is that two things – tectonics and the positions of continents, together with climate change and general cooling, led to the beginning of the Antarctic ice sheet about 34 million years ago, about at the boundary between the Eocene and Oligocene epochs of the Cenozoic. It’s had a long time to build up its ice – much longer than the ice ages that happened in the past 2 million years or so. —Richard I. Gibson Australia-Antarctic separation  West Antarctic Rift System Trans-Antarctic Mountains  Map from NASA, annotated by Gibson (generalized, based on various sources). 

There are lots of “theres” among the Cenozoic mammals. Brontothere, Uintatherium, Chalicothere, Indricotherium – the “there” or “therium” suffix is just Greek for “beast,” so it gets attached to the scientific names of lots of critters, normally to mammals.  image by DagdaMor, used under Creative Commons license. Mammalian groups that began in the Paleocene or early Eocene diversified and thrived during the Eocene, Oligocene, and later epochs of the Cenozoic. Paraceratherium – which may be the same as Baluchitherium or Indrichotherium or both – was the largest land mammal that ever lived. It was related to rhinoceroses but had a long neck, giraffe-like in its proportions. There are no complete fossils, one reason we aren’t sure which name goes with it, but it’s estimated to have been 20 feet high and 27 feet long. It lived in Central Asia pretty much throughout Oligocene time, 34 to 23 million years ago. Paraceratherium means “near the hornless beast,” because it was similar to a contemporary hornless rhinoceros. The rhino family was much more diverse in the Oligocene than it is today. Even though the incomplete fossils mean we can’t really reconstruct it, there is no doubt that it was a huge animal that browsed on foliage in trees. The name titanothere – titanic beast – has been pretty much discarded in favor of brontothere, the “thunder beast.” Brontotheres were hoofed animals that were descended from the ancestors of horses, but if you met a brontothere on the street you’d probably call it a rhinoceros. They typically had horns and other bony protuberances on their heads and faces, and unlike rhinos whose horns are made of keratin, the same compound that makes fingernails and hair, brontothere horns were bone. Brontotheres appear to have become extinct just about at the end of the Eocene. Their demise is speculated to be related to the rise of grasses – tough plants that they could not adequately graze on and process – and the related increase in arid conditions as the Oligocene began.  Uintatherium skull (public domain)Uintatherium, the Uinta Mountains Beast, lived in what is now Utah in middle to late Eocene time, and it had a cousin that lived in China. It was another critter that looked a lot like a rhino but wasn’t all that closely related to them. With big bony bumps and saber-like canine teeth, it must have looked intimidating, but it was a herbivore. Uintatherium belonged to the mammalian order Dinocerata, a group that is entirely extinct.  We aren’t sure how the Dinocerata related to other mammals, but I think the favored position is that they branched from an ancestor that was also ancestral to the ungulates, the hoofed animals like horses. But that’s not certain. All the Dinocerata went extinct before the end of the Eocene. Changing conditions, together with competition from brontotheres may have eliminated them. But the brontotheres survived only a few more million years themselves before dying out about at the end of the Eocene. The Chalicothere, whose name means “gravel beast,” appeared during the Eocene, and survived right up until about 4 million years ago, a run of 50 million years, unusual for the early Cenozoic mammals which typically evolved but in many cases did not survive as a genus or even at the family or order level. Chalicotheres had short hind legs and long front legs, so they were probably knuckle walkers, like modern gorillas. They were the size of a horse, and ranged across Europe and North America as well as parts of Asia and Africa. It is supposed that they used their strong but short hind legs to rear up and use their long forelegs to strip vegetation from trees. Unlike its hoofed relatives, the Chalicothere had three claws on its feet. Gastornis (public domain) Birds too were becoming huge, occupying ecological niches that their cousins, the non-avian dinosaurs held until the end of the Cretaceous. Gastornis giganteus, which lived in very late Paleocene and early Eocene tim

Let’s take a break today from mammals and talk about an impact in Chesapeake Bay about 35 million years ago, a few million years before the end of the Eocene Epoch. The area was offshore then as it is now, but the climate was probably at least sub-tropical if not tropical. Map from USGSThere is no doubt that this impact devastated the coastal region of the mid-Atlantic states. There’s some speculation that the impact-generated tsunami, which affected hundreds of miles of coastline, even overtopped the Appalachian Mountains in what is now Virginia. The center of the crater is pretty near the southern tip of the Delmarva Peninsula, but it has a diameter of 85 kilometers, 53 miles, making it the largest known impact structure in the United States and among the largest in the world. The crater was unknown until 1983, because it is entirely in the subsurface, buried by later sediments. It was suspected because of the discovery of fused glass shards and shocked quartz grains in an exploratory oil well in New Jersey, and it was confirmed and defined in the 1990s though additional drilling. Once it was known, it became clear that its presence in the subsurface actually affects the courses of some modern rivers, including the York and James Rivers, which turn sharply at the buried rim of the crater. It also affects modern aquifer systems in the area, and the crater region is subsiding at a faster rate than the rest of the coastal zone, about 6 inches per hundred years. The crater also shows up in detailed gravity and magnetic surveys of the area.  Cross-section from USGSWhat was it? We don’t really know, so it is referred to as a bolide, a generic term for an extraterrestrial body, but it was probably a comet or asteroid about 4 km across. There’s another impact feature of about the same age offshore New Jersey that might be part of the same event. It is dated to Late Eocene time as well, but detailed studies have not been done on that one. —Richard I. Gibson LINKS and References East-coast aquifers The crater  Gravity and Magnetic expression  Crater map from USGS (public domain)  Cross-section from USGS (public domain) 

The Cenozoic really is the Age of Mammals. They rose almost meteorically after the extinction at the end of the Cretaceous eliminated the dinosaurs, which must have preyed on the mammals, but probably even more important, they occupied ecological niches that in their absence mammals could fill. And especially with the Paleocene-Eocene Thermal Maximum that we talked about the other day, mammals began to diversify dramatically, and to grow in size.  3-foot-high Coryphodon reconstruction (source)Coryphodon was a browsing animal that appeared in the Paleocene and thrived in the Eocene. It was common in North America, where its fossils have been found well-preserved in the Wasatch Formation of Wyoming and Utah, dating to about 57 to 46 million years ago. The Wasatch Formation is more or less between the Paleocene Ft. Union formation and its coals, which we mentioned on Dec. 4, and the Green River Formation that we talked about yesterday, but they all overlap to some extent in both time and space. Coryphodon ranged into what is now the Canadian Arctic, Europe, and China too. At about a meter high, it was one of the largest land animals at the time.   Its name, Coryphodon, means “pointed tooth.”  It used those teeth to crunch leaves and other plant matter in the swampy places where it lived a life that must have been similar to the modern hippopotamus, but it was not closely related to hippos. Paleocastor burrow (source)By Oligocene time, about 25 million years ago, mammals were clearly expanding into diverse ecological niches. Paleocastor – whose name means “ancient beaver” – lived in and around lakes and swamps in what are now Nebraska and South Dakota. They made corkscrew-shaped burrows as much as 3 meters deep, which were first identified as giant freshwater sponges, but eventually one was found with the ancient beaver within it, and the marks in the burrow were tied to the animal’s teeth used to excavate the hole. Paleocastor was the size of a large rat, but beavers and many other mammals had a tendency toward gigantism over the course of the Cenozoic. Castoroides was a beaver that lived during Pleistocene time, the glacial period, and became extinct as recently as 10,000 years ago. Castoroides grew to as much as seven feet long. Stylinodon, another Eocene critter, had a short, pug-like face. Its name means “ribbon tooth,” for a ribbon of enamel around its canine teeth. Strong claws have been interpreted to indicate that Stylinodon dug roots and tubers for its food, but the claws and forelimbs might have been used to pull down branches and strip them of fruits. The animal grew to as much as 180 pounds (80 kg). It’s been called “an aardvark with the head of a pig,” (Janis et al., 1998) but it really doesn’t have much in common with any modern mammal. It was extinct by the end of the Eocene, one of many variations that did not survive. * * * We haven’t had any geological birthdays in a while, but today we have two. Israel C. Russell was born December 10, 1852, in Garrattsville, New York. He had a wide-ranging career with the U.S. Geological Survey and the University of Michigan, and he’s probably best known for his studies in Alaska and Mono Lake, California, as well as glaciers in the U.S. Fielding Bradford Meek was born December 10, 1817, in Madison, Indiana. His career as a paleontologist took him all over the United States, and he’s perhaps best known for his work in California and in the Badlands of the Dakotas. —Richard I. Gibson Paleocastor burrow photo from University of Nebraska and Agate Fossil Beds National Monument. Used under creative commons license.  Coryphodon by Heinrich Harder (public domain) Stylinodon Evolution of Tertiary Mammals of North America: edited by Christine Marie Janis, Kathleen Marie Scott, Louis L. Jacobs, 1998, Cambridge University Press.

You recall that the supercontinent Pangaea began to have some cracks in it almost as soon as it was assembled back in the Carboniferous and Permian. The break-up really got underway in the Jurassic and Cretaceous, and in particular, India separated from Africa and from Australia-Antarctica. India began to move northward, opening what is now the Indian Ocean and closing the most recent incarnation of the Tethys Ocean, sometimes called the Neotethys, in Late Cretaceous time. India’s northward drift from the old East Gondwana was remarkably fast, at rates close to 20 centimeters, 8 inches, per year. That’s not quite ten times the more typical rate of sea-floor spreading.  Sea-floor spreading rates are affected by the geometry of mid-ocean ridges and by the chemistry of the basaltic material brought up – sometimes it is somewhat denser than normal, or less dense. It may have something to do with the strength of a mantle heat plume, and spreading rates are definitely not uniform all along a spreading center at a mid-ocean ridge. In the case of India, differential rates meant that as the Indian block moved, it rotated a bit. I don’t think we have a clear idea of why India moved so quickly. But we do have a pretty clear idea of what happened when it reached another strong continent. The Himalayas happened.  India probably began to collide with Eurasia about 55 million years ago, about the boundary between the Paleocene and Eocene epochs. One line of evidence for this is the change in the spreading rate – it slowed dramatically from almost 20 cm per year to more like 4 or 5 cm per year. The collision may have slowed it down. The collision is still going on, 55 million years later.  from USGSWhen continents collide, their relatively low density, compared to oceanic crust, means that they do not subduct beneath each other, at least not much. But the crust does shorten, as the margins of the two colliding continents together with whatever was between them get scrunched together into huge folds and uplifts – and there is some pushing down, too, not truly subduction, but the crust on one continent may at least flex down under the weight of all the squeezed-together stuff. In India, that downward flexing created the Ganges Plain in northern India, just south of the Himalayan Mountain front. The India-Eurasia collision is so intense that the force is breaking the continent at great distances from the point of impact. Much of southeast Asia is being pushed out from between the two colliding continents, and 1000 kilometers or more inland, tension has broken the crust of Asia to form the Lake Baikal rift in Siberia. The high mountain relief of the Himalayas is the result of this squeezing. Present-day measurements of the rate of uplift, the highest on earth at about 10 cm per year in places, show that the collision is still continuing, and the earthquakes in the region are of course evidence for it as well. The Himalaya also has one of the highest erosion rates on earth, no surprise, at 5 to 10 cm per year. So erosion is pretty close to keeping pace with the uplift. Map of Alpine Orogeny elements by Woudlouper, used under Creative Commons license. India may have been the speediest part of Gondwana to charge headlong into Eurasia, but it wasn’t the only part. The northeastern prong of Africa, today’s Arabian Peninsula, was toward the narrow, western end of the Tethys Ocean. Tethys had already been narrowed somewhat by the movement of the Cimmeride continental blocks across it – we talked about that back in the Permian and again in the Triassic.  The Cimmerian blocks had amalgamated to the southern margin of Eurasia to form much of what is Iran, Afghanistan, and Pakistan today. Some other Cimmerian blocks were probably caught in the big squeeze between India and Asia, and are now somewhere within the Himalayas. The parts further west, Iran and vicinity, were collided with by the Arabian edge of Africa, and that collision is s

To start with today, an update on Archaeopteryx, the first bird. Or maybe not. New research is showing that the transition from dinosaurs to birds was complex, and some lines of thought suggest that Archaeopteryx was a feathered, gliding dinosaur rather than a true bird. This year’s Society of Vertebrate Paleontologists’ meeting in Berlin included new studies of Archaeopteryx. Here's a link to a report from the journal Nature on current thinking about the possible first bird. * * * Our Cenozoic topic today is the climate at the end of Paleocene and beginning of Eocene time, about 55 or 56 million years ago. Just about at the boundary between the two epochs there was a short, intense, 170,000-year period of warming called the Paleocene-Eocene Thermal Maximum, or PETM. Temperatures rose quickly, over about 20,000 years, by about 6°C, and carbon isotope ratios suggest that the oceans underwent acidification because of increased CO2 that was dissolved in ocean water. Sea levels rose because of thermal expansion of the water. Some extinctions are associated with the Thermal Maximum, especially among deep-water marine microorganisms, but some shallow-water calcareous microorganisms actually became more abundant. There’s no evidence for significant extinction on land, and in fact shortly after it began, there is a dramatic radiation of some mammals such as camels, horses, pigs, primates, and other groups.  We don’t really know what caused the changes in CO2 and temperature during this Thermal Maximum. There are some volcanic eruptions that are at essentially the same time, including some in Canada and Greenland that might have accounted for the spike in carbon dioxide, but the volcanism is more suited to explaining the more gradual warming that was occurring through the Paleocene. Spikes are always challenging to explain. Speculations about impacts have found no evidence. One good possibility is that warming oceans reached a threshold temperature at which methane hydrates – essentially, methane gas trapped in ice within sea-floor sediment – melted and released their methane, a more potent greenhouse gas than carbon dioxide. Such a release could have occurred over a short period of time – hundreds to thousands of years – making it a reasonable candidate for the spike in temperatures. But changes in carbon isotope ratios suggest that the effects of the event worked their way from shallow waters into deep waters over a span of around 10,000 years, the opposite of what would be expected with a deep-water methane release. Exactly what was going on in the oceans is hard to quantify – things that might support the methane idea include changes in ocean circulation that might have mixed warm tropical waters more thoroughly in the world ocean. The Isthmus of Panama did not yet exist, so there would have been more connection between the Atlantic and Pacific. The Paleocene-Eocene Thermal Maximum is being studied quite intensely because it may serve as a model for the modern increases in temperature and carbon dioxide that are occurring as a result of human activities. I have a link below to an article in Paleontology Online by Phil Jardine, which is a nice overview of the facts of the Thermal Maximum at the end of the Paleocene. —Richard I. Gibson Paleocene-Eocene Thermal Maximum

Everyone knows grass. Common worldwide today, grasses include the cereal grains such as rice and wheat, bamboo, swamp sedges, and of course lawns, but as common as they are today, grasses were the last major group of plants to evolve. They were certainly present but uncommon during the Cretaceous – dinosaur coprolites, or fossil excrement, are known with grass components. But grasses really began to expand during the early part of the Cenozoic, the Paleocene and Eocene epochs. Grasses continued to evolve through the Cenozoic, inventing novel ways to fix carbon during photosynthesis. Grasses including maize or corn, sugar cane, and sorghum use a more efficient method of carbon fixation than many plants, a method that is thought to be a relatively recent development – meaning probably the past 40 million years.  Grasses diversified a lot in the middle to late Miocene, around 6 to 10 million years ago. Their dominance in prairies and savannahs may be a result of their drought tolerance and ability to use carbon dioxide more efficiently than some other plants, even in low CO2 conditions such as were developing over the course of the Cenozoic. Another factor might be a co-evolution with hoofed animals that grazed on grasses, and helped spread them.  Coal mining, Powder River Basin, Wyoming (USGS photo)In the western United States during the Paleocene, as grasses were beginning to expand, swamps and lakes were forming in what are now northeastern Wyoming and southeastern Montana, the Powder River Basin. This basin was one of the low-lying areas between two of the late Cretaceous Laramide uplifts, the Black Hills and Big Horn Mountains. Those mountains were actively uplifting into the early Paleocene, and shedding sediment into the adjacent swampy basins, where extensive forests grew in Paleocene and Eocene time. The package of rocks including fluvial or river-borne sediments, lake deposits, and organic material deposited in swamps is called the Ft. Union Formation. The climate was warm temperate to subtropical but with alternating warm and cooler intervals during the Paleocene and Eocene, the first epochs of the Cenozoic. The plant matter in the Ft. Union Formation has produced thick coal beds. One individual bed, near Gillette, Wyoming, is about 110 feet thick. Similar coals of Cenozoic age can be found throughout the Rocky Mountains, but the Wyoming-Montana coals are the most important economically. Wyoming is the leading state in the US for coal, with 338 million tons produced in 2013, 39% of the U.S. total. West Virginia, at #2, produced 11% of the total, which amounts to about 1.1 billion tons of coal for the whole United States. In other plant-related news, just this week a new discovery was announced of a carnivorous plant in Baltic amber, dating to about 40 million years ago, the Eocene epoch. LINK Today, December 4, is St. Barbara’s Day. She’s the patron saint of artillerymen, mathematicians, and miners. Geologists sometimes adopt her, usually as an excuse for a party. —Richard I. Gibson LINKS and References: Recent evolution of grasses Carnivorous plant Evolution of grasses  Cretaceous and Tertiary coals of the Rocky Mountains and Great Plains regions, by R. Flores and T. Cross, 1991, in GSA DNAG volume P-2, Economic Geology, U.S. Photo from USGS 

To begin today I want to mention a new podcast I’ve just found out about, called Evolution Talk. It just began in August 2014 and as the title suggests, it’s all about evolution, including Darwin and other scientists that formulated the theory. It’s produced by Rick Coste on a weekly schedule, and I recommend it to you. It’s on iTunes, and I have a link on my blog to it as well. You can find information at EvolutionTalk. Today’s topic is the recovery from the extinction at the end of the Cretaceous. Some of the great extinctions in earth history clearly impacted the planet for a long time. The Permian extinction in particular, together with the single-continent nature of Pangaea, seems to have resulted in an early Triassic that was pretty antagonistic for life for a few million years, at least.  Life seems to have recovered from the end-Cretaceous event relatively quickly, which intuitively could be seen to support the idea of a very specific event, the asteroid impact, as the primary cause.  One study led by researchers at MIT even suggests that the recovery of oceanic algae and cyanobacteria was underway as soon as just 100 years after the impact. Since those microorganisms are the base of the food chain, their recovery would have paved the way for the expansion of other life as well, but if you look at the recovery in other ways, it may have taken more time, even much more, a typical million years or so.  The argument has been made that survivors were animals that could forage on dead plant or animal matter – which was presumably in abundance – rather than those that fed on living plants. A study of the interlinked recovery of plants and insects, led by researchers at Penn State, found that plant diversity was very low for 800,000 years after the extinction event, and for 8 million years, generally, plants and insects remained relatively low in diversity. But there were some exceptions where plants and bugs thrived, quite early in the Paleocene, the epoch immediately after the extinction. That kind of makes sense to me, that there might have been refuges, or locations where the devastation was not so intense, that could maintain or re-develop their diversity relatively quickly. However long the recovery took, it certainly did happen. As the phytoplankton in the oceans recovered, so did the surviving bony fishes. On land in the course of the 10 million years after the extinction, the Paleocene epoch, mammals and birds both diversified and became larger than their Cretaceous ancestors. It would appear that being small was no longer such an advantage, with the big predatory dinosaurs gone. By the end of the Paleocene, 56 million years ago, and into the following Eocene Epoch, some flightless birds were growing to six feet in height. Mammals mostly were still small compared to later in the Cenozoic, but there were several different mammals that reached a meter or more in length. I think it is clear that they were getting bigger, and we’ll talk about some of the mammals and birds of the Cenozoic later this month. I have links below to reports on the nature of the recovery from the end-Cretaceous extinction. —Richard I. GibsonLINKS: Chaotic recovery? Fish diversify Rapid recovery?  Recovery intervals Recovery dynamics  Paleocene mammals 

The Tertiary and Quaternary, the original periods of the Cenozoic, were divided into epochs by Charles Lyell in 1833, and the names he gave are still retained, although they are now assigned to the Paleogene, Neogene, and Quaternary Periods.  He used the same root word, “cene,” meaning recent, that is the prefix of the era, Cenozoic, but Lyell used it as a suffix when he named the epochs. He had analyzed assemblages of fossils from the different Cenozoic formations, and using the percentages of species that still were alive at present, he assigned names to the time periods that were marked by increasingly modern or recent fossil assemblages. He named the Eocene, which means “dawn-recent,” for rocks with fewer than 5% of modern species present, Miocene, “less recent,” for those that had 20-40% of modern species, and Pliocene, “more recent,” for those that were 40% to 90% modern. Since Lyell’s time, we’ve added Paleocene, “ancient-recent,” with essentially no modern species, Oligocene, “little or few-recent,” Pleistocene, “most recent,” and Holocene, “entirely recent.” In order from oldest to youngest, the epochs are Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene. There’s been a lot of discussion about naming a new epoch, the Anthropocene, which means “human-recent,” for the last few thousand years, but the Holocene is only about the past 12,000 years so there would be considerable overlap. In my opinion it’s all semantics and preference and doesn’t matter. The Paleocene, which began with the end of the Cretaceous, and the Eocene and Oligocene make up the Paleogene Period. The Miocene and Pliocene are the Neogene Period, and the Pleistocene and Holocene are the epochs of the Quaternary Period. The boundary between the Paleogene and Neogene is put at 23 million years ago, and the Quaternary started 2.5 million years ago and basically coincides with the glacial age. So, that makes the Paleogene about 43 million years long, pretty typical for a geological period, and the Neogene is 21 million years long, which is on the short side. The Quaternary, at 2.5 million years, is an exceedingly short time span for a period, but it’s OK because we know so much about it, and because it does represent a time when things changed a lot, the modern ice ages. There is some argument as to whether the Quaternary should be seen as a full-fledged period or should be an epoch of the Neogene. All semantics and technicalities. For now I’m going with what I perceive to be the internationally recognized scheme that has the Quaternary as a period, not that it really matters. I realize that that’s a lot of jargon and detail. I’ll try over the course of the month when I use names like Oligocene or whatever to keep it clear where we are as we progress through the Cenozoic. Bret HarteLet me close with a poem by Bret Harte, author and poet of the California Gold Rush. His poem, To the Pliocene Skull, was written in response to reports in the daily press of 1868: "A human skull has been found in California, in the pliocene formation. This skull is the remnant not only of the earliest pioneer of this State, but the oldest known human being. The skull was found in a shaft 150 feet deep, two miles from Angels in Calaveras County, by a miner named James Watson, who gave it to Mr. Scribner, a merchant, who gave it to Dr. Jones, who sent it to the State Geological Survey. . . . The published volume of the State Survey of the Geology of California states that man existed here contemporaneously with the mastodon, but this fossil proves that he was here before the mastodon was known to exist.” Harte was clearly skeptical of this discovery, and rightfully so. To the Pliocene Skull – by Bret Harte (A GEOLOGICAL ADDRESS) Speak, O man, less recent!  Fragmentary fossil! Primal pioneer of pliocene formation, Hid in lowest drifts below the earliest stratum Of volcanic tufa! Older than the beasts, the oldest Palaeotherium; Older than the tr