1. Cyclothems record sea-level cycles. What is the lithologic and stratigraphic
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Question
1. Cyclothems record sea-level cycles. What is the lithologic and stratigraphic evidence that you can use to identify the regressive part of the cycle?
2. How might you explain the presence of thinner limestone and an increase in coal-bed number and thickness toward the east?
3. The vertical lithofacies changes reflect sea-level changes. How many transgressions can you identify in the vertical sequence in column B? What relationships did you use to identify the transgressions? (should be three transgressions)
4. Examine the fossil range distributions within this stratigraphic section. Which fossil taxon (taxa) can be considered to be the best guide (index fossil)
Snail A Snail B/Starfish A Snail C/Echninoid A ClamA/Echinoid B Clam B
5. Using the criteria presented in the introduction, why do you think that these fossils are the best guide (index) fossils?
6.Using the criteria presented in the introduction, which fossil taxon is probably the worst guide fossil?
Snail A Snail B/Starfish A Sail C/Echinoid A Clam A/Echinoid B Clam B
7. Why is this fossil is not a good guide fossil?
Explanation / Answer
1. The energy at a point on the continental shelf is a function of the water depth. Therefore, with any change in sea level, the water depth at a point on the shelf will change introducing a new energy regime. This means a different facies will be deposited. Marine Regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously exposed land.Regressions may be caused by tectonic events such as orogenies, severe climate change such as ice ages or isostatic adjustments following removal of ice or sediment load.
In this case we will have deeper sea sediments (shales and limestones) being deposited bottom of continentally-derived beach sediments (sand). This forms a sequence (from top to bottom) of: sand ? shale ?limestone. A maximum regression occurs where the coarse sediments reach the farthest landward.
2.As transgression increases from west to east so limestone & coal bed thickness increased because A Marine Transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking or the ocean basins filling with water (or decreasing in capacity).In this case, sea water rises farther up onto land than it did before. In this case we will have deeper sea sediments (shales and limestones) being deposited on top of continentally-derived beach sediments (sand). This forms a sequence (from bottom to top) of: sand ? shale ?limestone. A maximum transgression occurs where the finest sediments reach the farthest landward.
3. Two/Three transgressions can be identified (from lithofacies change ) in coloum B. In coloum B we can see fining upward sequence (sanstone in bottom & limestone on top) & amount of limestone increase & coal bed is seen so drfinitely sea level increases & transgression occurs.
4. & 5. A good index fossil is one with four characteristics: it is distinctive, widespread, abundant and limited in geologic time. Because most fossil-bearing rocks formed in the ocean, the major index fossils are marine organisms.
Echinoids have lived in the seas since the Late Ordovician, about 450 million years ago. The living representatives of echinoids are the familiar sea-urchins that inhabit many shallow coastal waters of the world.Echinoids are marine animals belonging to the phylum Echinodermata and the class Echinoidea. They have a hard shell (test) covered with small knobs (tubercles) to which spines are attached in living echinoids. The test and spines are the parts normally found as fossils.Echinoid is considered as best index fossil because in this stratigraphic sections this is wide spread,abundant & limited in particular geologic time & also marine organism. (even Starfish is another type of Echinoids which is also present here).
6.&7. Snail is the worst index fossil here because Echinoid,Starfish & Clam have marine origin & habitat.Only Snails are terrestrial. In this stratigraphic section transgression & regression is found which is marine event so, terrestrial snails can not show many evidence about it.
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