1 / 4

How easily do grains round and spheritize (is that a word?)

How easily do grains round and spheritize (is that a word?). What do roundness & sphericity tell us about transport history and environment of deposition? Form (sphericity):

liliha
Télécharger la présentation

How easily do grains round and spheritize (is that a word?)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. How easily do grains round and spheritize (is that a word?)

  2. What do roundness & sphericity tell • us about transport history and environment of deposition? • Form (sphericity): • a) Sand size & smaller – mainly a function of breakage from parent material and mineralogy of grain. Form not altered significantly by transport. However, form has a great effect on settling velocity and the ability of a grain to stay in suspension. So downstream changes in shape do occur. • b) Pebbles & Cobbles – altered slightly more during transport. However, it is still not clear whether long- term abrasion has as measurable an effect as selective sorting. Ex. Prevalence of disk-shaped pebbles on a beach. Why? Abrasion by surf or sorting by surf?

  3. What do roundness & sphericity tell us about transport history and environment of deposition? 2) Roundness: A function of grain size, composition, transport process and distance. a) Sand size & smaller – Round less than pebbles & cobbles. Wind is 100 to 1000 times more efficient in rounding than water. Well rounded sand sand likely has some eolian history. b) Pebbles & Cobbles – Strongly controlled by pebble composition. Soft minerals like calcite round much faster than hard ones like quartz. Generally, Beaches round more than rivers. Wind doesn’t move large grains. Not a good measure of transport distance or depositional environment!

  4. What does surface texture tell us about Environment of Deposition? • Surface textures more easily modified than roundness or sphericity. Thus, features may reflect last round of transport & deposition – better indicators of ancient sedimentary regimes. BUT, many textures are produced in more than one environment! • Characteristics • Eolian (Wind) – Upturned parallel plates. Silica solution & precipitation features. Grains appear “frosted”. • Beach & Near Shore – “V” shaped percussion marks. • Glacial – Conchoidal fracture, parallel to sub-parallel striations

More Related