Marine Geo Exam #3

Why does West US has thinner sediment

Subduction Zones and Plate Boundaries (active margins) 

Side with thicker sediment

passive margin (sed can just sit and pile up)

Passive margins are the sides ______

relative to spreading beginning of rifting that leads to the mid ocean ridge 

Long time + slow strain

cooling of lithosphere 

Rifting and Basin Type are grouped based on

strain rate (deformation) v. stretch factor (how stretched the basin becomes) 

proto-oceanic troughs (when new crust is formed)

• melting of the crust

• Fast strain over a short period of time (does not allow for rapid cooling and magma can form under to melt the crust)

• Failed rifts = slow strain over a longer period of time (cools)

• Cooling prevents the rifting from going to completion

Distal tensile force

far field plate motion causing extension 

Thermal upwelling

upwelling mantle plume raises the lithosphere. Uplift causes the lithosphere to extend. Heating lowers the viscosity of the upper mantle. 

Stretching Mechanisms

• Pure Shear: body is elongated in one direction and shortened in the perpendicular direction by equal amounts 

• Simple Shear: parallel plates maintain their distance from each other while translating relative to one another

Mantle rocks are ____ than crustal rock

stronger (more heat required)

Asymmetric Conjugate Margins

used to be connected, focusing of the asthenosphere (hot mantle) leads to asymmetric rifting. Focusing results from detachment fault growth. 

Rifting Rates

Slow at first because of high strength of lithosphere, rifting accelerated as lithosphere thins and hotter mantle rock upwells. 

Magma-rich v. magma-poor passive margin

• Volcanic -> magma came out at the surface as rifting was occurring 

  • Seaward dipping reflectors -> numerous originally flat laying lava flows that are now tilted toward ocean basin 

  • Almost symmetrical rifting 

Basin infill

• clastic sediments: Control on sediments fed to basins is rivers draining into margin

• Evaporites: Shallow sea incursions into basins can be cut off from oceans. Seas dry up leaving evaporite deposits. Salt is less dense and wants to float leading to the sale diapirs 

Rifting and basin formation is driven by

changes in far-field plate motion and/or thermal upwelling

Rifting is relatively fast when

The upper lithosphere becomes weakened.

Magma-rich margins are marked by

strong seaward dipping reflectors

 (SDR) in seismic studies.

Carbonate Seds + Rocks

• Majority are calcium carbonate

• Biological in origin 

• Can form wave resistance structures (reefs) 

• Can be altered by disgenesis

Common calcareous organisms

• Corals, bivalves (clams), gastropods (snails); shallow water

• Marine algae (Halimeda); shallow water

• Plankton (foraminifera and coccoliths); deep water

Limits of coral reefs

• habitat is limited by the depth at which sunlight can penetrate (the euphotic zone) 

• Clear water and warm water 

• Too warm of water will cause bleaching as the zooxanthellae is expelled 

• 30 N to 30 S 

Marine carbonate factory

• Highstands lead to submergence and deposition (Factory on) 

• Lowstands lead to emergence and erosion (Factory off) 

Chemical erosion

• Carbonate is saturated in ocean water above the CCD, but sea level fall may expose reef to air, where rain and CO2 form carbonic acid that dissolves exposed carbonate

• Subaerial chemical erosion leads to karst features, a sure sign that carbonate system has been exposed above sea level

Carbonate Margin Geometry

• Silicalastic shelf -> broad, flat- topped, reef-rimmed platforms 

• Rimmed platforms -> which are not cemented or weakly cemented, and thus susceptible to wave action.

• Carbonate Ramp -> terrigenous sediment supply vs. carbonate production not as flat as the siliciclastic shelf

Future of Reefs

• 70- 90% of coral reefs will disappear within the next 20 years

• By 2100 few to no reefs will remain(?)

Calcareous Sediments

become sediment and become limestone

Coral

cnidarians (animals), they’re colonial (usually) polyps that build calcium carbonate structures to live on and provide protection.

Reef Building Corals

mutualistic relationship with photosymbiotic algae called zooxanthellae. 90% of organic material zooxanthellae goes to the coral to build a reef

Types of reefs

fringing, patch, barrier, atoll, bank or platform reef

Atolls form when

volcanic islands sink

Mo’orea

fringing reef on an eroding volcanic island (stage before atoll)

Atolls Drown

coral can not keep above and whole area sinks -> guyots

Layers you may expect for a Guyot (bottom to top)

Basalt (intrusive dikes and sills or extrusive), harder carbonates, shallow water limestone (shallow water w/ some roots), rusdist bivalves (barrier/fridging reef), atoll summit karst features (dissolved calcite) exposed to the air and rain water (erosion) (carbonate is killed by sinking or rising sea level), foraminifera sand

Fringing Reef

a reef system that grows fairly close to (or directly from) the shore, with an entirely shallow lagoon or no lagoon at all, dominated by massive branching corals

Highest (Reef) Energy

at the margin

Carbonate Factory

Siliclastic margins depend on continental sediment supply; carbonate systems produce their own sediment. Carbonate systems build vertically to match sea level and shed carbonate debris

Erosion

Carbon is produced and erosion processes are closely balanced where production is only slightly ahead.

Mechanical erosion

mostly on ocean side (driven by waves), grow but in a very energetic environment (generates carbonate debris -> sediment)

Biotic Erosion

organism break apart calcium carbonate material to get at the organic material contained inside

Carbonate Platform Sequence Stratigraphy

controlled by sea level, times of rising sea level carbonate growth is high, when sea level falls carbonate growth is low

Highstand Shedding

When the carbonate factory is “on” during sea level rise and high stand, high carbonate production means there is a lot of material that can be eroded (or “shed”) off the platform and into the surrounding deeper waters. Areas adjacent to carbonate platforms, the sedimentation rate is high during sea level rise and highstand and low during sea level fall and lowstand.

Platform Drowning

sea level is rising too quickly for coral to keep up (either due to global sea level rise or local subsidence) then carbonate production stops and the platform is buried in pelagic sediment

How reefs can show sea level changes

Different species of coral occupy different depth habitats in the reef; fossils of these corals can be used to determine ancient changes in depth at the reef, which can be related to global sea level change or tectonic uplift.

Cambrian Reef

530 Ma, Archaeocyathids

Archaeocyathids

first biotic reef builders, filter feeders (like sponges), brief dominance (10-15mya)

Silurian-Devonian Reefs

Stromatoporids (calcareous sponges) and Tabulate Corals, Stromatoporids grew by forming big calcareous sheets up to 5 m in diameter

Permian Reefs

Reef-building brachiopods, Guadalupid sponges (calcareous) and tabulate corals

Cretaceous

Rudist bivalves, scleractinian corals

Paleogene Reefs

Small reefs (10s of km scale) composed of corals and coralline algae. Corals stressed by high temperatures (PETM, Early Eocene Climate Optimum), so larger benthic foraminifera became one of dominant carbonate producers

Modern Reef Types

dominated by branching corals, This may be due to the diversification of echinoids, chitins, parrotfish, etc. that are adapted to consuming corals and coralline algae

Threats to modern reefs

Heat (bleaching), ocean acidification, disease, pollution, invasive species

Coral Bleaching

When coral polyps are stressed (usually due to heat but also due to disease, increased sedimentation, hypoxia, etc.) they expel their zooxanthellae symbionts, which causes them to turn white

Ocean acidification

Decrease in ocean pH (and thus carbonate ion saturation) makes it harder for corals to build their skeletons and to repair damage decreased pH is associated with reduced density of coral skeletons, making them more susceptible to breakage and predation

Disease

results of virus, bacteria, fungi, Environmental stress (excessive heat, low oxygen, high sedimentation, declining pH, etc.) can make coral more susceptible to disease.