LECTURE 3: Depositional System: Alluvial Fans
I. Depositional System = consists of associated sedimentary environments
Facies = a group or related itemsLithofacies = a group or related rocks of a particular lithology
Facies Model = a general summary of a particular depositional system that is derived from ovserving the distribution of facies in many modern and ancient examples.
II. Alluvial Fan
1. alluvial from Latin "to wash against" vs. fluvial (Latin "river")
2. Typically found where constricted, steep channels come out of mountains onto a wide valley with low slope.
3. Bajada = a group of coalesced fans
4. Source area: Mass wasting (e.g. mud flows, debris flows, rock falls) leads to gravity flows (mass flows). Stream water can later rework the deposits out across the fan.
5. Role of flow expansion - as flow becomes unrestricted coming out of a canyon, it widens leading to shallowing of the flow which reduces shear stress (by reducing the H in rwgHsina). Also if slope is reduced coming out of a canyon then sina goes down as well.
6. Deposits: Fan shaped, distributary pattern of channels
|
PROXIMAL |
MEDIAL |
DISTAL |
|
Most gravity flows deposited (matrix-supported etc) River channels cut down into head of the fan, develops a single large channel cut. Coarsest grained, most poorly sorted deposits |
Fewer gravity flows deposited Channels start splitting farther down the fan |
Fewest gravity flows readh this area Several small, shallow channels Finest grained, best sorted deposits |
7. Recognizing alluvial fans vs. rivers in rock record.:
radial pattern, adjacent to mountain (or high area), abundance of gravity flow deposits, radial pattern, channels get smaller and more abundant downstream.
8. Prograding fans leads to a coarsening upward sequence.
9. Walther's Law = sedimentary environments found adjacent to each other at one time will be found in vertical succession in the stratigraphic record if no major unconformities (i.e. time missing) exists.
10. Examples of fan deposits: