Firm Grounds

Glossifungites

Incised Valleys

Lowstand Systems Tracts

Ravinement

Transgressive Systems Tracts

This is a marine-flooding surface that forms the first significant flooding surface in a sequence. The TS, in most siliciclastic and some carbonate successions, marks the onset of the period when the rate of creation of accommodation space is greater that the rate of sediment supply. It forms the base of the retrogradational parasequence stacking patterns of the Transgressive Systems Tract. In areas of high sediment supply, e.g. on rimmed carbonate platforms, the rate of sediment supply may keep pace with the rate of relative sea-level rise and thus the TS will mark a change from a progradational to an aggradational parasequence stacking patterns. The TS often marks the base of the most prominent onlap.

If no lowstand or falling stage systems tract facies are preserved above the sequence boundary, the TS may coincide with this boundary. A TS is often characterized by the presence of a surface marked by consolidated muds of firmgrounds or hardgrounds that are cemented by carbonates. Both surfaces are often penetrated by either burrowing or boring organisms. For instance Glossifungites burrows are found penetrating the firm grounds and are often filled by an overlying widespread winnowed, sorted and often conglomeratic ravinement sediment, or lag. Cemented surfaces may be colonized and bored by a Trypanites ichnofacies and infilled by the sediments associated with the base of the transgressive system tract and are often wave winnowed.

If the rate of sediment supply is low over the transgressive surface this may merge landward with the maximum flooding surface. When a TS extends over LST valley fill, the response on the resistivity log curve may show a small local increase resistivity followed by a low. This increase in resitivity is in response to the carbonate cementation of the hardground, while the low is associated with deposition of trangressive shales.