e upper canyon walls in the Grand Canvon of northern Arizona are made up of many
ID: 153684 • Letter: E
Question
e upper canyon walls in the Grand Canvon of northern Arizona are made up of many types of sedimentary rocks. Some of the sedimentary rocks include (1) lossiliferous limestone, (2) sandstone and mudstone with mud cracks, and (3) cross stratified sandstone (wind-blown). For each of the rock types listed in the previous sentence, list whether they were originally deposited in a marine or nonmarine environments. Also list what type of depositional environment i.e. rivers, lakes, deep marine, ete.) are recorded by these rocks. Hint: Use the underlined words above as clues together with the summary of depositional environments on Table 3 1. ThExplanation / Answer
1. Limestone is very common in shallow (<10m) marine conditions of tropics because carbonate saturation in water is highest in tropical to warm, temperate seas. In relatively deeper water (upto 3000m depth) Pteropod ooze, Cocoliths and Foraminifera are predominant. Between 3000m to 5000m Globigerina ooze dominates the ocean floor. So the limestones which are formed in these areas are mostly fossiliferous. Most of these (90%) limestones are mainly found in shallower parts of seas.
2. Desiccation and compaction of water-saturated muddy sediments produce a system of shrinkage cracks, which form a network and divide the surface into polygonal areas (rectilinear or curvilinear in plan view). These are called Mud cracks. They form mostly on subaerially exposed water-saturated sediment surfaces (e.g. surfaces of dried up ponds, coastal and inland sabkha, lakes and lagoons, abandoned river channels, flood plains and intertidal zones). Some may also form subaqueously due to increase in salinity.
3. There are many types of cross-startification structures can be found like—cross bedding, ripple cross lamination, flaser & lenticular bedding, hummocky cross stratification etc.
Cross bedding (abundant in sandstones) forms primarily by migration of ripples and dunes in water or air. In Aeolian environment (wind-blown) cross bedding exhibits inverse grading.
Ripple cross lamination has a general appearance of waves when viewed in outcrop sections. It forms during rapid sedimentation in fluvial floodplains, point bars, river deltas with periodic flooding, turbidite deposits.
Flaser beddings form in fluctuating depositional conditions with periodic current activity. Lenticular beddings form in environment which favour deposition of mud over sand.
Hummocky cross stratification has undulating sets of cross-laminae and form in fine sandstone to coarse siltstone under the action of waves.
Thus from the above discussion it can be concluded that the wind-blown cross stratified sandstones are actually formed in Aeolian environment---may be a Desert Environment is possible.
Marine Environmnet Non-Marine Environment 1. Fossiliferous Limestone 2.Sandstone & Mudstone with mud cracks(mostly) 3. Cross-stratified sandstone (wind-blown)Related Questions
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