LA BREA TAR PITS: The Gunk at the Center of Los Angeles In the very center of downtown Los Angeles,

LA BREA TAR PITS: The Gunk at the Center of Los Angeles In the very center of downtown Los Angeles, next to the County Art Museum, is one of the world’s ultimate fossil localities – the La Brea Tar pits. The pits are not natural pits, but are pits leftover from the mining of the “tar” (which goes by the scientific name of “asphaultum” and not-so-scientific name of “smelly gunk” and which technically isn’t tar at all) and from voracious excavations for the fossil bones found in the asphaultum pits during ~1913 – 1915. The “normal” appearance of the tars would be a seep within a vent, which as the asphaultum hardens, becomes a small mucky black mound. Where, might you ask, did this “tar” come from? The oil that is responsible for the asphaultum dates to ~10 – 2 million years ago when the region that would become much of Los Angeles was several thousands of meters beneath the Pacific. Sediments were deposited in this deep basin that included highly organic shales (an excellent source rock for petroleum constituents) and sands (making an excellent reservoir material for the petroleum when it is expelled from the shales during lithification). A thickness of some 3000m of these source rocks were laid down and buried by younger sediments – note: it takes ~a thickness of 5000m of deposits for petroleum to form during lithification because of the requisite temperatures (~80C or more) attained about this depth. Early photos of the area of Central LA and the tar pits show oil wells everywhere (http://tinyurl.com/2uroyy ). All well and good. Now how did the gunk get from that depth to the surface? As earthquake activity demonstrates, the geology of Los Angeles is an energetic place. Since the deposition and lithification of the sediments of the Los Angeles basin, literally mountains have formed, plates have moved, faults have cracked and ruptured. The sedimentary section containing the oil is now much shallower. And it is crossed by active faults that allow the seepage of the much lighter petroleum towards the surface. The seepage of asphaultum began to collect into mounds and water-covered pools about 40,000 years ago. The oldest carbon dates of plant matter found within the pits are 38,000 years in age, meaning that the oldest fossils within the pits are no older than this. These “tar” pits trapped any creature, great or small, that happened to get mucked down in them. Mammoths (as the statuary in the photo represent), saber-toothed lions, dung beetles, giant vulture-ish birds called Teratorns (http://tinyurl.com/o3yebrk ) are among the over a hundred species that became stuck, and their bones and shells preserved in the asphaultum. Their preservation defines what is probably the world’s best observatory of an ancient ecosystem – it’s a window on the Pleistocene. The excavations are never ending: in 2006 the County Art Museum began building a new underground parking lot that resulted in the discovery of sixteen new fossil deposits. The Paige Museum has planted a “Pleistocene Garden” on the park grounds recreating the vegetation of Los Angeles during the Ice Ages. Thus, the Pleistocene remains alive, with all its gunk and glory, in the heart of LA. Annie R Photo used courtesy of http://www.tarpits.org/la-brea-tar-pits For information touristic and geologic:http://www.tarpits.org/la-brea-tar-pits/faqs and especially http://www.tarpits.org/sites/default/files/Exploration%20Guide.pdfhttp://science.howstuffworks.com/environmental/earth/geology/la-brea-tar-pits.htmhttp://www.ucmp.berkeley.edu/quaternary/labrea.htmlhttp://archives.datapages.com/data/pacific/data/071/071001/87_ps0710087.htm -- source link

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