Local shoreface ravinement along part of the Galveston coast (Texas) is eroding older salt marsh-lagoonal muds as the shoreline migrates landward. The resulting stratigraphy will eventually place tidal flat and local shoreface ravinement along part of the Galveston coast (Texas) is eroding older salt marsh-lagoonal muds as the shoreline migrates landward. The resulting stratigraphy will eventually place shoreface and tidal flat sands over whatever is left of the mud facies.

A schematic representation of the baselevel model that makes a bold statement about shoreline trajectories and stratigraphic trends in terms of accommodation space and sediment supply.

Coarsening-upward shelf cycles (yellow arrows), each beginning with an MRS that signals the start of baselevel rise. The top of the upper cycle is truncated by fluvial, possibly tidal channels that eroded the normal regressive deposits on the exposed shelf – these are interpreted as lowstand channels. They contain abundant trough crossbeds in pebbly sandstone. The channels are overlain by a fossiliferous, pebbly sandstone, the base of which is the MRS, that in turn is overlain by thin, calcareous siltstone, the top of which is the MFS. Details of these surfaces are shown below. Jurassic Bowser Basin, northern British Columbia.

Crossbedded sandstone (at the hammer), deposited during regression, is overlain by a pebbly mudstone (dashed line) that contains abundant whole and broken bivalves, gastropods, ammonites, and encrusting bryozoa; this is the maximum regressive surface, the record of baselevel rise. The pebbly bed is conformably overlain by calcareous siltstone that in turn is overlain by shale and thin sandstone beds that form the basal part of the next coarsening-upward regressive package. The top of the calcareous siltstone is a maximum flooding surface (the end of transgression – the condensed stratigraphy between the MRS and MFS is the stratigraphic record of transgression. Jurassic, Bowser Basin, northern British Columbia.

A closer look at the regressive – MRS contact and the calcareous, pebbly mudstone-siltstone (the basal part of a condensed, transgressive package). Many of the fossil molluscs are intact; the bivalves are articulated and encrusted by bryozoa, indicating minimal reworking across the sea floor. Their abundance signals very low sedimentation rates during transgression. The pebbly mudstone is overlain by calcareous siltstone that is part of the transgressive package, overlain abruptly but conformably by coarsening-upward shale-sandstone – the contact is the maximum flooding surface (just out of the picture). Jurassic, Bowser Basin, northern British Columbia.

A panoramic view of maximum regressive surfaces for three 3rd-order sequences in the Lower Triassic Blind Fiord Fm, Sverdrup Basin. MRS1 is placed where sandstone beds become finer grained and contain more intense bioturbation than sandstones below the MRS. The decrease in sediment supply provided greater opportunities for grazing-burrowing critters on the sea floor. Although the MRS1 signals a shift in baselevel and a reduction in sediment supply, sand remaining in residence across the shelf is still available for reworking and deposition. The cessation of sand supply is marked by the relatively abrupt sandstone-shale contact immediately above the MRS1. MRS1 signals the end of the Griesbachian-Dienerian sequence, and the beginning of the shale-dominated Smithian sequence. MRS2 signals the end of the Smithian sequence and the beginning of the Spathian sequence. Smith Cliffs, Sverdrup Basin. Image courtesy of Ashton Embry.

Three high-order coarsening-upward shelf cycles at the top of the Blind Fiord Fm (Lower Triassic), with the maximum regressive surfaces (MRS) indicated at the top of prominent sandstone beds. The middle unit is about 3 m thick. Black arrows indicate coarsening- upward; yellow arrow is fining-upward. A maximum flooding surface is identified at the change from fining-upward to coarsening-upward trends. Smith Cliffs, Sverdrup Basin, Arctic Canada. Image courtesy of Ashton Embry.

Shelf mudrocks of the Tangahoe Fm. (Late Pliocene – 3-3.4 Ma,) were eroded during the last Interglacial transgression. The ravinement surface (SR-U), that here developed as a bedrock shore platform, is characterised by prominent gutters and potholes. The overlying brown pebble conglomerates and sands (Late Pleistocene Rapanui Fm.) were deposited about 120,000 years ago as the shoreline and shoreface migrated landward. The sandy lithologies contain ripples, lenticular and flaser bedding that are usually associated with current reversals across intertidal and shallow subtidal mud-sand flats and estuaries. South Taranaki coast, New Zealand.

A spectacular exposure at Yelverton Pass (Ellesmere Island) showing two unconformable shoreface ravinement surfaces that embrace an entire cycle of baselevel rise – fall – rise recorded in a thin deep-water shale of Norian age (Barrow Fm.). The lowest SR-U that erodes folded Carboniferous rocks was the culmination of a hiatus lasting 117 million years – ravinement removed much of the evidence for subaerial exposure. The SR-U between the Barrow shales and Heiberg Formation encompasses the Late Norian to Early Rhaetian (Late Triassic) and heralds an extended period of delta construction that continued into the Early Jurassic. Image courtesy of Ashton Embry.

In this Late Triassic succession, a resistant unit of micritic limestone (about a metre thick) overlies an unconformable shoreface ravinement (SR-U) that (probably in combination with the subaerial unconformity) removed late Ladinian to early Carnian strata – a hiatus of about 10 million years. The transgressive limestone (Gore Point Fm. Middle Carnian) represents accumulation on a shelf at a time when terrigenous sediment supply was very low. Interbedded shale-sandstone beds below the SR-U (Murray Harbour Group) also accumulated on a shelf but during a period of greater sediment supply. Esayoo Bay, Sverdrup Basin. Image courtesy of Ashton Embry.

A modern diastemic ravinement (SR-D) developing in an estuary along the south Auckland coast, New Zealand. Salt marsh deposits are gradually being eroded by encroaching intertidal and subtidal sands. The left image shows remnants of the salt marsh muds and dead or dying mangroves with roots exposed by erosion. The right image shows some detail of the eroded marsh deposits, where the roots of Salicornia and rushes are exposed. Most of the open holes were formerly occupied by roots that decayed once they were exposed. Excavations through these deposits show that about 90% of the salt marsh unit will be removed, leaving only a veneer preserved in the rock record.

A Lower Cretaceous succession in Sverdrup Basin showing the transition from subaerial exposure (SU – within the Isachsen Fm.) to transgression where excavation by shoreface ravinement (SR-D) was minimal. The intervening non-marine fluvial channel and carbonaceous floodplain deposits preserve the subaerial unconformity. The diastemic SR surface is placed at the dramatic facies change from non-marine sandstone, mudstone, and coal beds to outer shelf, marine shale. Buchanan Lake, Ellesmere Island. Image courtesy of Ashton Embry.

Panoramic view of maximum regressive and maximum flooding surfaces for three 3rd-order sequence in the Lower Triassic Blind Fiord Fm – the MRS surfaces are described under the maximum regressive surface banner.
The Smithian MFS (dashed line) is placed at the transition from fining- to coarsening-upward trends and indicates the end of transgression in this sequence. Shale below the MFS is organic-rich, reflecting low terrigenous sedimentation rates. In the overlying Spathian sequence, the MRS2 and MFS are separated by only 40 cm. In this case, the sequence boundary is located within a shale succession. Subtle, but important transitions like this require close examination of the outcrop or core to detect changes in stratigraphic trend. Smith Cliffs, Sverdrup Basin. Image courtesy of Ashton Embry.

Three coarsening upward pro-delta through delta plain cycles, each containing prominent maximum flooding surfaces in the Campanian portion of the Expedition Formation (Eureka Sound Group). Each cycle begins with an MRS that marks the top of normal regression (coarsening upward sandstone-shale – arrows) and the beginning of baselevel rise. The records of transgression in each cycle are thin, fining-upward packages about 50-80 cm thick, that culminate in an MFS at the transition to coarsening-upward trends of the succeeding regressive succession. Axel Heiberg Island, Arctic Canada.

Detail of parasequence stratigraphy from the Jurassic Bowser Basin, northern British Columbia. The top of the regressive succession is identified as an MRS surface, outlined by the yellow dashed line. Above is a pebbly, fossiliferous muddy sandstone (small ammonite indicated by arrow), that grades upward to grey, muddy, micritic limestone (a few pebbles). The limestone is very hard and fractures conchoidally. The limestone is overlain by another coarsening-upward shale-sandstone package (the regressive component of the next parasequence); the limestone-shale contact is a maximum flooding surface (MFS).

A classic topographic expression of resistant regressive sandstone cycles, each culminating in a maximum flooding surface in parasequences from the Jurassic Bowser Basin, northern British Columbia. The MFS in the foreground occurs just above the prominent bench where it is overlain by coarsening upward shale-sandstone of the succeeding regression. At least four such packages are present in this view (arrows indicate approximate MFS positions). Maximum regressive surfaces are also present below the MFS, as shown in the previous image. In each cycle, the transgressive package accounts for 10% or less of the total cycle thickness.