Peritidal and subtidal carbonate environments

Question 1

Define peritidal environment

Answer 1

Includes supratidal (usually emergent and occasionally flooded) and intertidal region (flooded twice a date by high tide, deposition when flooded, evaporation when emergent)

Question 2

What controls distribution of facies of peritidal facies?

Answer 2

Tidal range, wave action, climate, hydrology, topography

Question 3

How does tidal range affect distribution of peritidal facies?

Answer 3

Most peritidal facies develop where tidal range is low

Question 4

How does wave action affect the distribition of peritidal facies?

Answer 4

Usually in low-energy protected lagoons

Question 5

How does climate affect the distribution of peritidal facies?

Answer 5

Different facies mosaics under humid and arid conditions

Question 6

How does hydrology affect the distribution of peritidal facies?

Answer 6

Influences diagenesis in the supratidal zone

Question 7

How does topography affect the distribution of peritidal facies?

Answer 7

Can influence the development of island and lagoons

Question 8

What is the sediment source for peritidal facies?

Answer 8

Subtidal
Channels rework sediment and focus onshore sediment transport during storms

Question 9

Describe peritidal facies

Answer 9

Low faunal diversity, deposition of pelletal lime mud in channels/ponds, rootlets, burrows, algae

Question 10

Describe peritidal facies in the supratidal zone

Answer 10

Plant debris (algal mats and magrove rootlets), muddy sediment, desiccation cracks, reworking of lithified crusts by storms (e.g. mud chips, flat pebble conglomerates)

Question 11

Describe peritidal facies in the intertidal zone

Answer 11

Microbial mat (bc of flooding), high abundance, low diversity fauna, muddy, desiccation polygon, fenestrae, white layers of evaporites, bioturbation in lower intertidal

Question 12

Give four examples of depositional processes and sedimentary structures in arid climate peritidal facies

Answer 12

Dolomite, algal mats, desiccation cracks, wind-blown sand/mud

Question 13

How can depositional texture in arid climate peritidal facies be disrupted or obscured?

Answer 13

Early diagenetic evaporite precipitation

Question 14

Describe subtidal zone facies

Answer 14

Inc. ponds and tidal channels
High diversity, high abundance skeletal fauna
Bioturbation
Fining-upward successions, levee deposits

Question 15

How are platforms exposed?

Answer 15

Rainwater reacts with soil, forming mild acids, this percolates down, following pre-existing structural trends
In humid settigs, a dense network of interconnected caves is created

Question 16

Where do calcrete karsts form?

Answer 16

(Semi) arid climate

Question 17

What is observed in peritidal carbonates?

Answer 17

Cyclicity

Question 18

What are the two shallow subtidal environments?

Answer 18

Lagoons and open shelfs

Question 19

Describe the open shelf

Answer 19

No restriction, normal salinity
Abundant and diverse fauna does not tolerate salinity variation, therefore there are primarily brachiopods, echinoderms, bryozoa

Question 20

Describe the lagoon

Answer 20

Restriction, elevated salinity, stom recharge causes salinity fluctuations
Abundant, low diversity fauna tolerates salinity variation (gastropds, algae, ostracods)

Question 21

What do optimal conditions for fauna in shallow waters lead to?

Answer 21

Fossil-rich bioclastic limestones
Fossil content depends on salinity and substrate nature

Question 22

Give three examples of areas of high-energy and high carbonate production rates

Answer 22

Shoreface, shelf edge, areas of strong currents (tidal, storm)

Question 23

What are carbonate sand bars/shoals analogous to?

Answer 23

Siliciclastic shorelines and sand waves
Very clean, well-sorted carbonate sands
Porous, oolitic grain stones are effective shallow aquifers