Transgressive Stratigraphy at Sequence-Bounding Unconformities: Some Principles Derived from Holocene and Cretaceous Examples
Dag Nummedal, Donald J.P. Swift, 1987. "Transgressive Stratigraphy at Sequence-Bounding Unconformities: Some Principles Derived from Holocene and Cretaceous Examples", Sea-Level Fluctuation and Coastal Evolution, Dag Nummedal, Orrin H. Pilkey, James D. Howard
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Sequence stratigraphic concepts are powerful tools in the analysis of the evolutionary history of sedimentary basins. The criteria used in the identification of depositional sequences differ for outcrop and subsurface data sets because of scale differences in stratal continuity and spatial resolution. This paper documents sedimentary and stratigraphic characteristics of sequence boundaries and the overlying transgressive succession of depositional systems. By emphasizing the sedimentology and the different patterns of systems stacking above the sequence boundaries, this paper aids in identifying these key stratigraphic boundaries in outcrop.
The key stratigraphic surfaces encountered in shallow marine sedimentary sequences are: (1) subaerial unconformities, cut by episodes of sea-level fall, and representing a significant hiatal break; and (2) diastems, surfaces representing relatively short interruptions in sedimentation. Most diastems in shallow marine and coastal settings are associated with transgressions and may be loosely grouped as “transgressive surfaces.” Diastems include the ravinement diastem, which is formed by transgressive shoreface retreat, channel-base diastems, and various marine erosion diastems.
Subaerial unconformities of regional or interregional extent and their correlative marine unconformities and conformities serve as boundaries of depositional sequences. The depositional architecture between the sequence boundary and the overlying transgressive surface, e.g., the ravinement diastem, is controlled by the relief of the coastal plain and the rate of transgression.
Transgression of a coastal plain previously dissected by an episode of significant sea-level fall will produce shore-normal estuaries and shore-parallel lagoons. The resultant stratigraphic succession will be characterized by an inner-shelf sand sheet above back-barrier deposits and “ribs” of fluvial and estuarine sediments filling former subaerial valleys. This architecture characterizes the Holocene stratigraphy of the United States Atlantic and Gulf shelves, and the basal Cretaceous strata (Oak Canyon Mbr of the Dakota Sandstone) of the San Juan basin in New Mexico.
In rapidly subsiding basins, such as the Holocene Mississippi Delta region of the Gulf Coast basin and the western foredeep area of the Cretaceous Interior Seaway, relative sea-level fall may be insignificant or nonexistent. Consequently, the sequence boundary may be a rather subtle unconformity, or even a conformity. Nevertheless, the transgressions will cause erosional shoreface retreat, the cutting of ravinement diastems and formation of offshore erosion surfaces. These surfaces generally are of limited spatial extent and are discontinuous in the stratigraphic record, yet, they form distinct lithostratigraphic breaks and may easily be mistaken for sequence boundaries. Time lines cross these transgressive surfaces, but they do not cross sequence boundaries.
The Coniacian strata of the San Juan basin provide an example of a sequence boundary and associated transgressive depositional systems formed in response to a relatively small sea-level fall and subsequent rise in a rapidly subsiding basin. A mid-Coniacian sea-level fall, detectable across the entire Western Interior Seaway, caused the western coastal plain rivers to return to grade by depositing a coarse fluvial sandstone (Torrivio Mbr of the Gallup Sandstone), enhanced the bypassing of fines, and intensified storm-induced coarse-grained sediment transport onto the distal shelf (Cooper Arroyo and basal Tocito Sandstones). The subsequent transgression resulted in shoreface retreat by destruction of distributary mouth bars of the Torrivio Mbr, formation of offshore linear shelf sand ridges above the ravinement diastem (Tocito Sandstone ridges), the deposition of a series of transgressive sand bodies in back-barrier settings (e.g., flood-tidal deltas of the Borrego Pass Sandstone), and rapid accretion of low-energy fluvial facies (Dilco Coal Mbr of the Crevasse Canyon Fm). Individual ravinement surfaces underlie many Tocito Sandstone ridges and separate them from regressive facies, but these surfaces may be of limited areal extent and are probably not connected to a single regional unconformity. The ravinement surfaces and the transgressive Tocito sandstones climb progressively higher in the section relative to the mid-Coniacian sequence boundary as one moves updip (paleo-landward).
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Sea-Level Fluctuation and Coastal Evolution - This Special Publication is the result of a symposium in honor of W. Armstrong Price held at the first SEPM Midyear Meeting at San Jose, California, on August 12, 1984. The factors controlling relative sea-level change along our shores are varied and, at best, imperfectly understood. Yet, the relative rate of change is what controls shoreline erosion, the arrangement of sedimentary facies of the coastal zone, and the character of deformities within the coastal stratigraphic record. Therefore, these papers address sea-level changes, shoreline responses, and the controls on the three-dimensional geometry of the consequent lithosomes; in short, the architecture of the coastal depositional systems.