FALL 2004 – GLY 4552 Sedimentary Geology Lab Experiment

USING SEDIMENTARY FEATURES TO INTERPRET SEDIMENTARY ENVIRONMENTS- A Test of Textural Maturity

Introduction and Problem

One on the observations that sedimentologists use to interpret ancient depositional environments is the relative degree of sediment maturity.  Sediment maturity is a measure of distance or time from the source area to the depositional site. A lot of factors influence sediment maturity, especially the climatic condition of weathering and transport and the mineralogical make up of the source area rock. Maturity can be gauged in terms of texture (textural maturity), mineralogy (minerologic maturity), and composition (compositional maturity). The terminology used to describe maturity is relatively simple: immature, submature, mature, and supermature

Textural maturity is gauged largely in terms of grain size, grain sorting, and grain roundness. At a source site the weathering process tends to generate a wide range of grain sizes but generally big chunks. And these tend to have a rough or angular exterior. The maturation process makes big things smaller up to a point that is. Grains of medium sand size tend to resist further size reduction. The maturation process also knocks of the rough edges reducing the grain exterior to a smooth or rounded surface. Therefore:

 

Immature

Submature

Mature

Supermature

Sorting

Extremely poorly sorted to very poorly sorted

poorly sorted to moderately sorted

moderately well sorted to well sorted

very well sorted

Grains Size

very coarse or bigger

coarse

medium

Medium

Roundness

Angular

subangular

rounded

well rounded

The inherent assumption in using sediment maturity in interpreting depositional environments is the following:  “The greater the transport distance and length of time in the transport medium, the more mature a resultant sediment becomes”.  Note that this is a testable hypothesis established over the years based on many observations of field samples.  For the first part of the semester, you will test this hypothesis in lab using samples collected from two depositional environments in Alaska (Fig. 1).  Sediment in both settings has ultimately been produced through glacial erosion in the Chugach –St. Elias Mountains.  Fundamentally, glacial sediment is extremely heterogeneous, reflecting the fact that glacial erosion (e.g., plucking, abrasion) produces clasts of all sizes, from boulders the size of houses to rock flour smaller than 1 micron.  However, the mode of transport and time spent in transport is substantially different between the two locations from which I have chosen samples.  One of the sampling sites is from Disenchantment Bay (Fig. 2), where the sediment is deposited largely from melting of icebergs calved from the Hubbard Glacier.  The ice-rafted debris in these icebergs has experienced only englacial transport without any hydraulic sorting other than settling through the water column.  The sand from the second site on the Copper River delta (Fig. 3) also was produced by the same glacial erosion processes but has experience thousands of years of transport by moving water.   By using samples from the same weathering environment, we reduce some of the variables that influence sediment maturity.  Consequently, we can hypothesize that the textural maturity of the CRD sample should be much greater than the DB sample because it has experience more fluvial transport.  In the interest of time, we will not focus on compositional or mineralogic maturity because the two samples are from drainage basins with different source rocks, and so the composition of the sands will more likely reflect this fact rather than any difference in composition caused by chemical weathering during transport.

To test this hypothesis, you will be given FOUR unknown subsamples to analyze, two from each of the two sites.  In particular we will look at the degree of abrasion or roundness of a grain, the general shape of grains, and the grain-size distribution of each sample. Your objective here is to learn certain basic descriptive procedures for these grains and to explore just what one might be able to interpret from them. Through out this laboratory exercise and all subsequent ones keep in mind that it is very important to separate interpretation from description. Always observe, then describe, and finally then and only then interpret.

To help in your interpretation, we will pool the data from all the students to provide you with a larger sample size.  Ideally, you would want to analyze dozens of subsamples to examine for any natural variability in the sample populations.  In the world of statistics (which I’ve been accused of spending too much time), analyzing more samples increases your degrees of freedom, allowing you to make more statically robust interpretations about differences between your two environments.  Unfortunately, there is not enough time to delve deeply into the statistical analysis of your data.  Rather, I will do the statistics for you and you will use these results in your experimental report. 

To turn in: A final project report described here.

 

Some background information on Textural Maturity:

It has been observed that there are the following trends in texture as the time in transport increases:

– Grain size (see Wentworth grain size scale).

• Cobble, boulder, pebble, sand, silt and clay.

• Grain size tends to decrease as the time in transport increases.

– Grain shape (Fig. 4).

• Roundness: Degree of angularity of grains

• Sphericity: How well the shape approaches a sphere

• Both roundness and sphericity tend to increase with increasing as the time in transport.

– Grain sorting:

• Sorting refers to the range of particle sizes in a rock. Sorting tends to increase as the time in transport by water or wind increases.

 

 

Fig. 1.  MODIS satellite image from June 2004 showing the location of sampling sites in Southern Alaska

 

           

Fig. 2.  Sampling location of Disenchantment Bay (DB) sand.  An example of englacial sediment is shown on the right.

 

 

           

 

Fig. 3.  Sampling location of Copper River Delta (CRD) sand.  LEFT: Photo of Copper River from plane window; RIGHT: sample collection point on point bar

 

 

                                    

Fig. 4A-Hypothetical ice-rafted sand grain        Fig. 4B-Hypothetical glacifluvial sand grain