Chapter 4 Marine Sedimentation ©2003 Jones and Bartlett Publishers

Classification of marine sediments can be based upon size or origin. Size classification divides sediment by grain size into gravel, sand, silt and clay. –Mud is a mixture of silt and clay. Origin classification divides sediment into five categories: terrigenous sediments, biogenic sediments, authigenic sediments, volcanogenic sediments and cosmogenic sediments. 4-1Sediment in the Sea

Factors that control sedimentation include particle size and the turbulence of the depositional environment. Terrigenous sediments strongly reflect their source and are transported to the sea by wind, rivers and glaciers. Rate of erosion is important in determining nature of sediments. Average grain size reflects the energy of the depositional environment. 4-1Sediment in the Sea

Hjulstrom’s Diagram Hjulstrom’s Diagram graphs the relationship between particle size and energy for erosion, transportation and deposition.

Based upon water depth, the ocean environment can be divided into the shelf, which is shallow and near a terrigenous source, and the deep ocean basin, which is deep and far from a terrigenous source. 4-2Sedimentation in the Ocean

Shelf Versus Basin Depths

Shelf sedimentation is strongly controlled by tides, waves and currents, but their influence decreases with water depth. Shoreline turbulence prevents small particles from settling and transports them seaward where they are deposited in deeper water. Particle size decreases seaward for recent sediments. Past fluctuations of sea level have stranded coarse sediment (relict sediment) across the shelf including most areas where only fine sediments are deposited today. 4-2Sedimentation in the Ocean

Worldwide distribution of recent shelf sediments by composition is strongly related to latitude and climate. Calcareous biogenic sediments dominate tropical shelves. River-supplied sands and muds dominate temperate shelves. Glacial till and ice-rafted sediments dominate polar shelves. 4-2Sedimentation in the Ocean

Shelf Sedimentation Model

Geologic controls of continental shelf sedimentation must be considered in terms of a time frame. For a time frame up to 1000 years, waves, currents and tides control sedimentation. For a time frame up to 1,000,000 years, sea level lowered by glaciation controlled sedimentation and caused rivers to deposit their sediments at the shelf edge and onto the upper continental slope. For a time frame up to 100,000,000 years, plate tectonics has determined the type of margin that developed and controlled sedimentation. 4-2Sedimentation in the Ocean

Relict Sediment

If influx of terrigenous sediment is low and the water is warm, carbonate sediments and reefs will dominate. 4-2Sedimentation in the Ocean

Deep-sea Sedimentation has two main sources of sediment: external- terrigenous material from the land and internal-biogenic and authigenic from the sea. 4-2Sedimentation in the Ocean Sedimentation in the Deep Sea

Major sedimentary processes in the deep sea include:, Bulk emplacement, Debris flows, Turbidity currents Major pelagic sediments in the ocean are red clay and biogenic oozes. Authigenic deposits are chemical and biochemical precipitates that form on the sea floor and include ferromanganese nodules and phosphorite. 4-2Sedimentation in the Ocean

The distribution of sediments in the deep ocean reflects latitude, distance from landmasses, and the calcium carbonate compensation depth. Glacial marine sediments occur in the high latitudes. Pelagic clays occur far from land and in the deepest water. Calcareous oozes occur above the calcium carbonate composition depth. The rate of sedimentation depends on the type of sediment in deep sea. 4-2Sedimentation in the Ocean

Deep-sea stratigraphy refers to the broad- scale layering of sediments that cover the basaltic crust. The stratigraphy of the deep sea is strongly influenced by sea-floor spreading. 4-2Sedimentation in the Ocean

The Atlantic basin contains a “two-layer- cake” stratigraphy–a thick basal layer of carbonate ooze overlain by a layer of mud. 4-2Sedimentation in the Ocean Stratigraphy of the Atlantic Basin

The Pacific basin contains a “four-layer- cake” stratigraphy, because unlike the Atlantic its sea floor as it spreads crosses the equator where the CCD is lowered to the ocean bottom. 4-2Sedimentation in the Ocean

Stratigraphy and Model of Pacific Basin

Bottom dredges scrape the sediment and collect material in a wire or canvas bag. Grab samplers take a “bite” out of the sediment covering the bottom. Gravity and piston corers use a weight to drive a core barrel into a soft bottom. A piston corer takes a much longer core than a gravity corer because of the piston in the core barrel. 4-3Collecting Marine Sediment There are a number of sampling techniques for obtaining sediment from the ocean bottom.

The Mediterranean basin is located where plates are colliding as Africa moves northward relative to Europe. Anhydrite and stromatolites of Miocene age indicate that the Mediterranean sea “dried” out between 5 and 25 million years ago. Two models have been suggested to account for this emptying of the Mediterranean Sea of its water. –The “Uplift” Model –The “Drying-Out” Model 4-4 The Drying Up of the Mediterranean Sea

Refilling the Mediterranean Sea After drying out, seawater from the Atlantic Ocean cascaded down the face of the Gibraltar Sill, refilling it in about 100 years.