Olduvai Formation 7 - General Chronology and Stratigraphy of Olduvai Gorge

Now that you have familiarized yourself with the general course of the development of the Olduvai Gorge, we will introduce you to some more specific aspects of the chronology and stratigraphy.

The following cards contain diagrams and generalized profiles of the Olduvai Gorge stratigraphy. Where applicable, you may click on a stratum to see the developmental stage that corresponds to that layer in the sequence you just viewed. In addition, you may click on the symbols for the various stratigraphic layers in order to see a pop-up field with more detailed information about the stratum.

Although Olduvai contains archaeological materials covering most of the time span from 3+ million years ago to very recent times, in this exercise we are primarily interested in Bed I, which contains the oldest hominid fossils from the area.

Chart of the Chronology and Stratigraphy of Olduvai Gorge

This scale is calibrated to time in millions of years before present based on assays of the potassium-argon content of volcanic tuffs. Review the stack on dating techniques if the K-Ar method is not clear to you.

Polarity refers to whether the polarity of the Earth's magnetic fields was Normal compared to today, or Reversed. Periods of Normal Polarity are marked in solid black. Periods of Reversed Polarity are white.

Polarity Event simply refers to named episodes of deviations in the Earth's magnetic polarity in relation to the general polarity trend for that epoch.

A Polarity Epoch is simply the name given to a period of time with a well defined long-term trend in the polarity of the Earth's magnetic field. We are currently in the Brunhes Normal Epoch.

Stratigraphic Unit refers to the names given to broadly categorized groups of geological strata that are in a clear chronological relationship to each other.

General Stratigraphy of Bed I at Olduvai Gorge

Lake Deposits

The lake deposits consist of fine-grained clays and mudstones deposited at the bottom of the large lake that formed in the basin sometime during the developmental history of Bed I, and continued into the period when Bed IV was formed, at which point it completely dried up. The Lake Deposits are interbedded with lava and alluvial fan deposits on the east due to recurrent changes in the extent of the lake. The interbedding with the alluvial plain deposits on in the west is due to this same phenomenon.

Lake-Margin Deposits

The Lake-Margin deposits are a complex group of clays, siltstones, mudstones, and conglomerates derived from the constant ebb and flow of the lake's edges and changes in the rate of erosion and deposition from the areas surrounding the lake. It is the lake-margin deposits that have proven to be the most productive in terms of the preservation of evidence of hominid activity.

Alluvial-Fan Deposits

The Alluvial-Fan deposits are a complex of eroded, redeposited volcanic tuffs, ashes, and mud-flows carried by stream action down from the volcanic highlands to the south and east of the original gorge basin. These materials eventually formed a huge alluvial fan spreading out from the highlands and down to into the original basin, displacing the lake by several miles. A number of the tuff strata are quite distinct, and have proven useful for dating the stratigraphic sequence in the area using the Potassium-Argon method. Tuffs Ia and Ib are the lowest strata for which reliable K-Ar dates have been obtained.

Lava Flows

The lava flows exposed along the eastern portion of the gorge underlay, and are considered a part of Bed I. These flows appear to derive from a period of very intense volcanic activity sometime prior to about 2 million years ago. The majority of the flows came from the Olmoti volcano, directly east of the gorge. In the southern portion of the original basin, the lava flows probably came from a vent on the side of the Ngorongoro volcano, although the vent has yet to be located. There is some indication that the lava flows initially displaced the eastern lake margin, but were then covered by the lake later as the water level rose.

Alluvial-Plain Deposits

The Alluvial-Plain deposits are derived from sediments brought down into the basin from the south and west by stream activity during the period prior to and after the formation of the lake. They interfinger with the lake deposits on the western end of the gorge.

Welded Tuff
The Welded tuff strata all probably derive from massive eruptions of the Ngorongoro volcano sometime during the early part of the Cenozoic era. On the west the tuff has been called the Naabi Ignumbrite, and is clearly contemporaneous with similar deposits found further south at Laetoli. The tuff on the eastern end of the gorge has a slightly different chemical composition, and so cannot definitely be assigned to the Naabi, but they did derive from the same volcano, possibly as the result of an earlier eruption.

Basement Metamorphic Rocks

The Basement Metamorphic rocks constitute the lowest known member of the stratigraphic units at Olduvai. These rocks consist of gneisses and quartzites of Precambrian age that have been folded and warped by the tectonic displacement of the Afro-Arabian craton, of which they are a part.

Detail of the Gross Lake-Margin Stratigraphy

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