Sedimentary structures of the Sydney Basin

The Sydney Basin has many examples of sedimentary, bedding, and stratigraphical structure.
Below are examples from a number of formations to assist with identification.

Where these rocks come from

Most of the structures shown here come from the Wilton Formation, part of the Illawarra Coal Measures, exposed in the coastal platforms and cliffs at Austinmer and Wombarra Beach.

The Wilton Formation rests disconformably on the Erins Vale Formation beneath it. A disconformity is a break between two parallel sets of beds where the older rocks were exposed and eroded for a long period before the younger ones were laid down on top: the layers run parallel, but a great deal of time is missing between them.

Together the two formations record a shoreline moving back and forth. The Erins Vale Formation is sandy shelf sediment laid down as the sea advanced; the Wilton Formation above it is upper shoreface and beach, along a storm-dominated coast fed by meandering rivers. Within a few metres of cliff the outcrops pass through distal marine, shallow marine, embayment and peat mire settings — the peat mires being the swamps that became the coal seams.

That is what makes this stretch of coast so useful. Each of those environments leaves its own signature in the rock, and here they are stacked close enough together to compare side by side.

Laminations

Lamination can occur as parallel structures (parallel lamination) or in different sets that make an angle with each other (cross-lamination). It can occur in many different types of sedimentary rock, from coarse sandstone to fine shales, mudstones or in evaporites.

Dropstones

Dropstones are commonly interpreted as being dropped into the sediment from floating icebergs, ice rift etc and dropped in vertically through the water column. Evidence includes an impact depression beneath the dropstone and angular clasts. These dropstones were deposited from season icemelts in a fluvio deltaic setting.
Formation: Wilton Formation, NSW Australia

Bioturbation

Bioturbation is the reworking of soils and sediments by animals or plants. Its effects include changing texture of sediments, bioirrigation and displacement of microorganisms and non-living particles. Fluvio deltaic setting.
Formation: Wilton Formation, NSW Australia

Load Structures / Load Casts

Load structures or load casts (also a type of sole marking) are blobs that form when a denser, wet sediment slumps down on and into a less dense sediment below.
Ball-and-pillow deformations are a result of a physical shock that has been applied to unconsolidated sediment. This shock causes rupturing to occur in the sedimentary rock layer, which induces instability. Individual lobes caused by this unstable state break off and move downward, settling into the underlying layers. The "pillows" that have now been produced typically retain their original bedding compositions. earthquakes, erupting volcanoes, or meteoric impacts can create these formations.

Planar laminae

Beds that are deposited in even, horizontal sheets are called plane beds and are composed of planar laminae, typically 5 - 20 grains in thickness. Plane beds form at high flow velocities, beyond those capable of forming dunes. Also typical of plane beds are current lineations, which are furrows and ridges a few grains in height that are oriented parallel to flow direction.
laminae do vary in thickness laterally.

Graded Bedding

In geology, a graded bed is one characterized by a systematic change in grain or clast size from the base of the bed to the top.

Herringbone cross-stratification

Herringbone cross-stratification is type of sedimentary structure formed in tidal areas, where the current periodically flows in the opposite direction.

Ironstone

Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement. The iron minerals comprising ironstones can consist either of oxides, i.e. limonite, hematite, and magnetite; carbonates, i.e. siderite; silicates, i.e. chamosite; or some combination of these minerals.

Iron oxide concretions

Iron oxide concretions or Hematite nodules are often formed in sedimentary rocks by oxidation of pyrite or marcasite crystals.

Petrified wood

Petrified wood is the name given to a special type of fossilized remains of terrestrial vegetation. It is the result of a tree or tree-like plants having completely transitioned to stone by the process of permineralization.

Syneresis Cracks

Syneresis cracks are formed by the contraction of clay in response to changes in the salinity of a liquid surrounding a deposit.

Ossillating Wave Ripples

Also called bidirectional ripples, or symmetrical ripple marks have a symmetrical, almost sinusoidal profile; they indicate an environment with weak currents where water motion is dominated by wave oscillations.
Formation: Wilton Formation, NSW Australia

Mudcracks

Mudcracks (also known as desiccation cracks or mud cracks) are sedimentary structures formed as muddy sediment dries and contracts. Crack formation also occurs in clayey soils as a result of a reduction in water content.