Students begin leveled profile exercise atop the cut-bank above GaJi12.

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Geological Features at the Koobi Fora Spit Site (GaJi12)

Sediments exposed in the beach on the south side of Koobi Fora Spit from 1985 to 1996 were finely layered beach and near-shore deposits of fine and medium sand, silt, clay silt and silty clay. Most of these sediments are grey in color and contain features that reflect near shore environments. It is these features that are the subject of this web page.


Fossil ripple marks at Koobi Fora Spit.
 

  Ripple marks preserved in a thin layer of silty clay that overlies the slightly-cemented, oxidized yellow-brown sands of a beach that was once emergent for a brief period. It is these layers of oxidized sands that are associated with human activity on the Spit. Note that the ripples are parallel with the axis of the spit, suggesting a simple foreshore rather than a long-shore lagoon.

  When the resistant layers of silty clay and slightly cemented sand are breached, wind erosion quickly eats through the underlying grey beach sands.


Dimpled wave marks at Koobi Fora Spit
 

When the lake level rises seasonally, ponds form behind the beach berm. Eventually, the berm is breached, creating a long shore lagoon. Waves are refracted from the shore around the mouth of the lagoon, creating a complex pattern of dimples and cross-ripples. This picture shows such a set of dimples formed in a thin layer of silty clay overlying slightly cemented, oxidized, yellow-brown sands.

About 150 meters distant, this is probably the same layer as that shown in the image above.


<i>Melanoides</i> marking the burried depression of a <i>Tilapia</i> nest on Koobi Fora Spit.
 

  The white shells are fossil Melanoides. Their circular pattern marks a filled-in Tilapia nest. The nest was a bowl-shaped depression formed in near-shore, lake bottom silt. After it was abandoned, it was lined with snail shells that drifted across the bottom and settled in the depression. The nest was then filled with silt and the associated surface buried. When the lake fell, a wave-cut, ripple-marked surface formed, truncating the nest and exposing the shells. Fine wind-blown sand fills the ripples.


Fossil <i>Tilapia</i> nest, Koobi Fora Spit.
 

  Tilapia nest excavated through beach sands, lined with a thin layer of clay, and filled with silt. Exhumed by wind erosion, which has removed the silt, leaving the more resistant clay. The brown and black bits are fossil fish bones concentrated on the surface by deflation.


Fossil <i>Tilapia</i> nest in silt at Koobi Fora Spit.
 

  Another fossil Tilapia nest escavated into silt and then filled with silt. Only the lining of Melanoides shells reveals the presence of the nest. In this case there are two arcs of shells laid down at two different stages in the filling of the nest. The black objects are fossil fish bones. Artefacts are seldom found on surfaces such as this; when found, they are almost always rolled.


Fossil fish vertebra <i>in situ</i>, Koobi Fora Spit.
 

  Looking down on a fossil fish vertebra about as big as the end of your thumb. The vertebra is in situ in a thin layer of silty clay that caps underlying beach sands. See the image below for stratagraphic context.


Fossil fish vertebra <i>in situ</i>, Koobi Fora Spit.
 

  Stratigraphic context for the fossil fish vertebra seen in the image above. From bottom to top:
1) transgressive phase grey beach sand; 2) oxidized yellow-brown sand of exposed beach; 3) coarse grey regressive beach sand; 4) medium grey regressive beach sand; 5) grey silty clay of a deeper water regressive phase. Vertical relief is about 15 cm.

  Oscillations of this sort are common in the beach sediments on the south side of Koobi Fora Spit.


Sub-fossil of Crock bone <i>in situ</>, Koobi Fora Spit.
 

  Crocodile bone in situ in medium grey beach sand. Notice the fragmentation in situ and the diagonal rampart of sand holding the bone in place. This specimen has been completely undermined by wind erosion and is gradually breaking along pre-existing fractures and falling to the deflation surface. This particular bed of sand does not contain artefacts or the butchered remains of animals, so this crock bone is just a sub-fossil on its way to erosional oblivion.


Ancient terrestrial beach with cultural material, Koobi Fora Spit.
 

  Amid the eroding lacustrine deposits and their many fossils, emergent beach horizons are briefly exposed before being deflated by wind erosion. These horizons are made of oxidized, yellow-brown sands that are frequently cemented just enough to resist erosion for a brief time, forming shelves above shallow blowouts such as you see here. Not the dark, silty clay horizon that covers this horizon on the left, the casts of roots from shore-side vegetation, and the wing of the vertebra that is protruding from the base of the sandy layer. The top of the vertebra is just barely exposed below the white end of the marker.

As you can see from the down-stream striations, the wind has been exposing this surface from right to left. Where the striated surface ends, note the hummock-like topography of garyer colored sand. This is wind-blown sand that is accumulating in the bottom of the blow-out. It is in such blow-outs that most cultural materials are found, with small numbers of specimens recovered from the yellow sand horizons at blow-out margins.


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© 2008/2009 by Charles M. Nelson
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