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1999 Field Season: Day 6
June 30, 1999
Excavations continued today in the same four sites as yesterday. With all equipment and vehicles back in camp and functioning correctly (for the time being), we were full staffed and expect to make decent progress if we can continue in this manner. We thought that a brief primer of the geology of the Olorgesailie Basin would be helpful, especially as we are always referring to "Member 11' " or "the redbeds of Member 8."
First off, the Olorgesailie Formation is a group of sediments that were deposited in the Olorgesailie Basin between one million and two hundred thousand years ago. The Formation is dominated by lake deposits with river channel and soils also preserved. Within the Olorgesailie Formation (hereon we will refer to it as "OF") there are thirteen distinct units that we call the "Members" of the OF. They are numbered one through five, and seven through fourteen. That is, on the original maps of the area, the geologists noted fourteen members, but in the course of our research here at Olorgesailie, we have determined that the unit called Member 6 is really nothing more than a localized part of Member 7 that looks slightly different. An easy mistake to make on the first map. For simplicity's sake, instead of renumbering all the other Members, we have chosen to drop Member 6 from the list. The upper two members (13 and 14) have yet to be studied in detail by our team, so we won't be including a description of them here.
Member 1: A series of lake deposits and"paleosols," or ancient soils, intermixed with tuffs, which are compacted layers of volcanic ash. The Elephas recki from Site 15 was found in one of the soil layers at the top of Member 1. One of the lower ash layers, coded Tuff A-5, has been dated to 992,000 years ago, which allows us to date the Elephant Site to around 990,000 years ago.
Member 2: Clean "Diatomites." A diatom is a microscopic animal that makes a shell or "test" out of silica (the same material quartz is made from). Diatomite is sediment created from the tests of diatoms. It is usually very light when picked up, and crumbles easily in your hand. Diatoms are aquatic, and indicate that the basin was covered in a lake at this time.
Member 3: A "massive" tuffy-diatomite. This means that tuffs and diatomites that were originally intermixed have been reworked, or eroded and redeposited together. The result is a thick grayish layer.
Member 4: "Pumice tuffs" with channel sandstones. Pumice is a volcanic product, a dark gray rock with many air-filled voids. The tuffs of Member 4 are filled with pumice, and are quite distinctive from the lower tuffs. Channel sandstones are sandstones formed from ancient rivers flowing through the deposited tuffs. They are bright yellow sandy layers amidst the dark gray tuffs, and stand out in stark contrast.
Member 5:A poorly-developed paleosol layer that has a small pumice layer that was dated to 974,000 years ago. This means that the many meters of sediments from the A-5 tuff of Member 1 to Member 5 were deposited rapidly in roughly 18,000 years.
Member 7: Another distinctive channel sandstone with a well-developed, or "mature" paleosol on top. In contrast to the rapid deposition below, a few meters of sediment were deposited here in over 200,000 years. We can date the top of this layer to 747,000 years ago. The Member 7 beds are the most famous of the OF, being the layer where high concentrations of hand axes and baboon fossils were discovered.
Member 8: A "redbed" diatomite. In contrast to the normally bleached white color of the diatomites in the basin, the Member 8 diatomites are bright red, indicating a possible high temperature burning event. We are still unsure as to how this event could have taken place, and are looking for clues in the rock record.
Member 9: Diatomite and tuff couplets. Up to three couplets of volcanic tuff and diatomite layered one on top of the other. The couplets contrast well against each other, and so this member is easily recognizable when seen in cross-section.
Member 10: A pumice tuff that has the texture of cheap styrofoam. It is coarse to the touch, very light when held, and crumbles easily. The contact between Member 10 and the overlying Member 11 is one of the more distinct divisions in the entire OF. The pumice in Member 10 has been dated to 662,000 years ago.
Member 11: This member of the OF is broken into three sub-members, denoted 11', 11'', and 11''' ("prime," "double-prime," "triple-prime"). The three layers are diatomaceous (containing diatoms) beds, with intermixed paleosols, and sand layers. The sand layer that we are attempting to excavate at Site A11-10 is at the top of Member 11'. Some fossils and artifacts have been found scattered through sand layers in Member 11" as well.
Member 12: A pumice rich layer with some interbedded diatomite, and in localized places, red-bed diatomites. The pumice of this layer in some parts can be rather large and rounded, giving it a distinctive "pumice marbles" appearance. This pumice is 601,000 years old.
Excavation will continue, and we should be ready to set up our microfauna analysis project by tomorrow.