Lecture 5 - Terrrestrial Facies Models Flashcards
Definition of facies
“the sum total features such as sedimentary rock type, mineral content, sedimentary stuctures, bedding characteristics, fossil content etc. Which characterise a sediment being deposited in a certain enviroment.
Facies models explanation
Observe moden proccess to refer to the acient records
What are the process
1) wave and current actvity
2) Debris flow
3) sea level cjhange
4) biochemical precipiation
5) bioturbidation
What are the products of these in the same order
1) Ripples
2) graded bedding
3) grain size
4) mineralogy
5) trace fossils
whats key in this
No two depositional settings are identical, therefore we can predict the old setting but looking at the modern day processes.
Why use facies models
These can be used to predict the setting of these rocks, and thereofre can be used for obvious advantages in economic and engineering geology.
Alluvival fans as an example of a facies model
allival fans form when a narrow canyon stream enters a flat valley. Cone shaped despoists of coarse stream sediments, sheet flood deposits and debris flow. it requires constant uplift
example of an allivial fan
Death Valley
What is the tectonic setting of an aliuvial fan
Areas undergoing rapid uplift
Geometry
Wedge shaped, limited in lateral extend
Sedimentology
Clay to boulders, particle size decreases towards to edge of the fan. Debris flow is unsorted and reverse grading.
Fossils present?
Rare
What is the acient example of an alluvial fans
Hornelen basin, Norway
Example 2: Braided rivers
Network of small braching channels often seperated by islands. occour where there is not enough engery to carry sediment load. Found near the source of a system.
Braided rivers techtonic setting
upper reaches of allival fans, near uplift
Geometry
Elongated, straight or sheet like sand bodies with cross cutting channels. Grade laterally into finer desposits.
Sedimentology
Sand dominant Gravel commment in longtitudal bars Very little silt Abudent tabular and cross cut stratification High verlocity plans in longtitdual bars Ripples and dune formation common
Fossils?
Rare, possible vegiation
Example 3: Mendering rivers
Lower reaches of the fluvial system, erosion on the outside and depoisition on the inside. This leads to lateral accretion of point bars.
Tectonic setting
lower reaches of the fulival system
Geometerry
long ribbon like bodies with sand scattered randomly in a thick sequence
Sedimentology
Gravels to pebbles.
Laterally increasing point bar sand (oldest on the oustide of the depoistion side)
Fossils
wood on a flood plain, water molcuse
Example 4: Lake deposits
Landlocked body of standing water, with small tide and small waves. Found in regions of tectonic depression, ie volcanic crators. to be preserved thick sequences must develop before evaporation
Tectonic sessint
Downward basins
Geometerry
Lenticular in cross section and circule or long in birds eye view
Sedimentology
Laminated mudstones on the lake bed. MARLS carbon rich mud. Fresh water limestones. Hypersaline lakes: evaporuites and mud
Fossils
frequent. Fish and insects, absense of marine fossils is a good indicator of thses lakes.
