June 21. Mississippian amphibian tracks

The Mauch Chunk formation in Pennsylvania, Maryland, and West Virginia contains relatively coarse clastic sedimentary rocks – clastic means broken, and the broken pieces were sand grains, silt, and pebbles that became sandstone, siltstone, and conglomerate. The rocks are mostly reddish, indicating oxidized iron and indicating an environment that was alternately under water and exposed to air. The most likely depositional setting was a delta or the low shore of a shallow sea, or possibly a swamp that was periodically inundated and then drained. Some of the rocks are green, indicating reduced iron, which forms in the absence of oxygen in a setting like a swamp.  

The beds are late Mississippian, about 320 to 340 million years old, but the youngest layers may be from the early Pennsylvanian. The coarser sediments also tend to be from the upper part of the formation and may reflect the onset of the Alleghenian or Appalachian Orogeny, the “big crunch” of mountain building that represents the collision of Gondwana with eastern North America. That collision was probably underway by about 325 million years ago, near the end of the Mississippian.

from Bronson, 1910

One of the coolest things about the Mauch Chunk is that it contains indications of land-walking animals, very likely amphibians. Footprints of tetrapods, four-footed animals, have been found impressed in sandstone with ripple marks. The ripple marks mean the sediment had been under water recently, but the well preserved footprints were made in the sand when it was exposed to air – they would not have been so well defined underwater.

In 1908 E.B. Branson and his colleagues from Oberlin College found such prints in late Mississippian rocks in Virginia. The animal’s feet were about 6 centimeters long, about 2½ inches, and they had five toes, at least some of which were webbed. The animal was probably lizard-like, although in the early discoveries they found no drag marks from tails.

In addition to footprints, impressions of nearly complete animals have been found. There’s a link on the blog to a 2007 report on those body impressions. The report draws possible conclusions about mating behavior based on the nature of those impressions.

The name Mauch Chunk is from Native American Lenape words meaning “bear place”. Today the community that once had that name is called Jim Thorpe, Pennsylvania, for the Olympic athlete.  

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On June 21, 1887, Norman Levi Bowen was born in Kingston, Ontario. N.L. Bowen was an experimental petrologist who studied the ways minerals crystallize from molten magma. Much of his work was at the Geophysical Laboratory at the Carnegie Institution in Washington, D.C. He established Bowen’s Reaction Series, an explanation of how various minerals form based on the temperature of a cooling magma. The reaction series helps explain why some minerals are commonly associated with each other while others, such as quartz and olivine, are rarely if ever found together. It also has implications for the stability of rocks and how they are likely to erode and alter under weathering conditions on the earth’s surface or during later metamorphic events.

Today is also the anniversary of a devastating earthquake in 1990, in northwestern Iran. The death toll was 40,000 to 50,000, and several large cities and many villages were almost completely destroyed. The quake was centered in the western Alborz Mountains along the Caspian Sea, where a relatively small continental block is trapped between the colliding Arabian Plate to the south and the Eurasian Plate to the north. The southern Caspian Sea is a small bit of dense oceanic crust, a remnant of the Tethys Ocean that lay between Gondwana and Eurasia. The microcontinent that forms today’s Iran is being pushed up and over that little oceanic crustal fragment. This has uplifted the Alborz Mountains – and puts volcanoes in them – while the Caspian Sea and most of its margins are below sea level. The high point in these mountains, Mount Damavand, is about 5600 meters or 18,000 feet above sea level, and it rises to that height within about 80 kilometers or 50 miles from the below-sea level coast, making this one of the steepest mountain fronts in the world.

—Richard I. Gibson

Amphibian fossil body impressions (2007 report)

Photo taken from Journal of Geology, v. 18, #4, p. 357, 1910, report by Branson (volume in author’s collection).