The sequence boundary is an unconformity updip and a correlative conformity downdip. Where it is an unconformity, it is a surface of subaerial exposure and erosion; however, the expression of those features in an individual outcrop may or may not be obvious. In places, an unconformity may be marked by obvious erosion, such as a major incised channel or a bevelling of structurally tilted underlying strata. Regionally, unconformities may display up to tens or sometimes hundreds of meters of relief. In siliciclastic systems, this relief is generated principally by downcutting rivers. In the undissected regions between rivers, called interfluves, paleosols may mark an unconformity, and their presence may be indicated by caliche nodules or rooted horizons.
Downdip at its correlative conformity, a sequence boundary is commonly marked by an abrupt basinward shift in facies. This abrupt shift is called a forced regression by some workers to distinguish it from a normal regression in which a shoreline moves seaward simply due to sedimentation. An abrupt basinward shift of facies is manifested in an outcrop by an abrupt shallowing, such as shoreface sediments directly overlying offshore sediments or mid-fan turbidites directly overlying basinal shales. As facies above and below such a basinward shift in facies commonly represent non-adjacent environments, this surface is abrupt and Walther's Law cannot be applied across it. Minor submarine erosion may be associated with this abrupt basinward shift of facies. Farther downdip, the correlative conformity may display no obvious facies contrast or other unusual features; the position of the sequence boundary in these cases can only be approximated.
Sequence boundaries are generated by a relative fall in sea level. As this is a relative fall in sea level, it may be produced by changes in the rate of tectonic subsidence or by changes in the rate of eustatic rise, as long as those changes result in a net loss of accommodation space. Early models of sequence boundary formation argued that the sequence boundary formed at the time of maximum rate of fall, but subsequent models suggest that the age of the sequence boundary can range in age from the time of maximum rate of fall to the time of eustatic lowstand.
The lowstand systems tract is commonly capped by a prominent flooding surface called the transgressive surface. The transgressive surface represents the first major flooding surface to follow the sequence boundary and is usually distinct from the relatively minor flooding surfaces that separate parasequences in the lowstand systems tract.
The transgressive surface may be accompanied by significant stratigraphic condensation, particularly in nearshore settings, which may be starved of sediment because of sediment storage in newly formed estuaries. Typical features indicating condensation are discussed in more detail below.
Following the relatively low rates of accommodation during the lowstand systems tracts, relative sea level begins to rise at an increasing rate. When this long-term rise is coupled with the short-term rise that forms a parasequence boundary, a major flooding surface is formed. The first of the series of these flooding surfaces is called the transgressive surface. In updip areas characterized by subaerial exposure and erosion during the lowstand systems tract, the transgressive surface and sequence boundary are merged into a single surface. Such situations are common in slowly subsiding regions such as in cratonic regions and the landward areas of passive margins.
Maximum Flooding Surface
The maximum flooding surface caps the transgressive systems tract and marks the turnaround from retrogradational stacking in the transgressive systems tract to aggradational or progradational stacking in the early highstand systems tract. The maximum flooding surface represents the last of the significant flooding surfaces found in the transgressive systems tract and is commonly characterized by extensive condensation and the widest landward extent of the marine condensed facies.
Condensation, that is, the preservation of relatively long geologic timespans in a relatively thin layer of sediment, can be indicated by many sedimentary features. Condensation or slow net deposition allows more time for diagenetic reactions to proceed, so condensed sections are commonly enriched in normally rare authigenic minerals such as glauconite, phosphate, pyrite, and siderite. Carbonate cementation is allowed more time to proceed and hardgrounds may form, and these may be subsequently mineralized with iron, manganese, and phosphorite crusts, as well as become bored or encrusted by organisms. The slow accumulation of sediment allows more skeletal material to accumulate and condensed sections may be indicated by unusually fossiliferous horizons or shell beds. Likewise, slow rates of sediment accumulation allow burrowing organisms more time to rework a given package of sediment, so burrowed surfaces are common in condensed sections. Slow rates of accumulation allow normally rare materials like micrometeorites and volcanic ashes to accumulate in greater abundances. Shales at condensed sections are commonly radiogenic as a result of increased scavenging of radioactive elements from the water column; such 'hot shales' display a strong positive response on gamma ray logs.
Sediment starvation is not the only process leading to slow accumulation rates, and many condensed sections are characterized by sediment bypassing, in which sediment is either moving through the system as suspended load or as bedload. When sediment moves through as bedload but fails to accumulate significantly, the condensed section is commonly characterized by numerous internal erosion surfaces and can have a quite complicated internal stratigraphy.
In outcrop, the maximum flooding surface is recognizable by the deepest water deposits within a sequence. In cross section, the maximum flooding surface is marked by the farthest landward extent of deep-water facies. In distal areas where the transgressive systems tract is absent, the maximum flooding surface may merge with the transgressive surface.
Early models of sequence stratigraphy argued that the maximum flooding surface coincides roughly with the most rapid relative rate of sea level rise, after which sea level rise begins to slow. Subsequent models have demonstrated that the maximum flooding surface corresponds more closely in time with the highest stand of eustatic sea level, rather than the time of maximum rate of rise.
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