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Mulholland 1998 - Parasequences
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T he parasequence is one of the fundamental stratal units in sequence stratigraphy. Although individual parase- quences are generally below the resolution of seismic lines, sets of stacked parasequence are distinguishable by their characteristic stratal patterns. Geologists working with well logs base much of their interpretation on the recog- nition of individual parasequences, their vertical and lat- eral facies relationships, and their stacking patterns. Thus the parasequence is of prime importance in recognizing sequences and interpreting depositional and tectonic his- tory from sequence development. The term parasequence itself is a product of the devel- opment of the principles of sequence stratigraphy. Repetitive, shallowing-upward stratal units were usually known as cycles, especially in the field of carbonate facies geology. The word cycle implies a repetitive time series, and seemed inappropriate for repetitive rock units. Van Wagoner introduced the term parasequence in 1985 at the SEPM midyear meeting and later wrote: “This usage pre- served the dictionary use of the word ‘cycle’ by Vail et al. (1977) to indicate a time in which a regularly repeated event occurs and emphasized the relationship between the parasequence and the sequence.” It was the development of the concepts of parasequence stacking patterns as building blocks of sequences that made them all-important. The fact that parasequences stack in orderly and predictable patterns controlled by rel- ative sea level enhanced prediction of depositional envi- ronments and made subsurface correlation of time and facies more reliable. Improved correlations produced bet- ter reservoir models, new exploration plays, and more success for the upstream oil and gas industry. In a previous article, I discussed the architecture of sequences, or the manner in which parasequences com- bine in predictable patterns to form sequences. This arti- cle will look in more detail at the parasequence itself, the operative developmental processes, the vertical and lateral facies relationships that result, and the depositional envi- ronments represented. The best guide to this information is still AAPG Methods in Exploration Series, No. 7, Siliciclastic Sequence Stratigraphy in Well Logs, Cores, and Outcrops, by J. C. Van Wagoner, R.M. Mitchum, K.M. Campion, and V.D. Rahmanian. This report will be mostly a synopsis of this excellent 1990 publication. The three fig- ures in this article also come from that source. Characteristics. Parasequences have been identified in all coastal environments where sea-level variation and sedi- ment variability are sufficient to produce recognizably dis- tinct facies. Most parasequences are progradational, and all exhibit shoaling-upward features. A siliciclastic parase- quence is essentially a miniature highstand systems tract terminated by a marine flooding surface (abrupt increase in water depth). While a systems tract is composed of parasequence sets, the parasequence is composed of bed sets. It might be argued that if the upper boundary (the flooding surface and the thin strata just below it) were examined closely enough, it would have all the charac- teristics of a sequence boundary, complete with uncon- formity, lowstand, or shelf margin systems tract, and a transgressive systems tract. The flooding surface in effect is thus a “maximum flooding surface” and the shale deposited atop the parasequence is a miniature condensed section. In certain situations the erosional surface associ- ated with the marine flooding surface (usually in the prox- imal region of the parasequence) has been called a transgressive surface of erosion. Further detailed outcrop studies will be required to resolve the true nature of this surface, but the fractal nature of sequence architecture favors the idea that this erosion is the result of sea-level regression rather than transgression. In a significant but little known 1974 paper (Texas Bureau of Economic Geology Circular 74-1), David Frazier pointed out the (now) obvious, that the sediments for parase- quences are delivered by rivers to the coast, and parase- quences build seaward and fill the basin from the shore toward the center. Prior to this, most geologists who hadn’t carefully thought the problem through envisioned basins as filling from the middle or from the middle toward the shore. As a case in point, it was widely accepted that transgressive sheet sands were deposited along the shore- line as it migrated landward with the transgressing sea. We now know that there is really no such thing as a trans- gressive sheet sand. It is actually a series of sand bodies deposited as a retrogradational parasequence set, now called a transgressive systems tract. While all parasequences record shoaling-upward con- ditions, they do not all necessarily display coarsening- upward sediments. Van Wagoner and colleagues illustrate three basic parasequence stratal patterns: the beach parase- quence (Figure 1) and the deltaic parasequence (Figure 2), which are similar and coarsen upward, and the tidal flat parasequence (Figure 3), which is fining-upward and com- monly terminates with a coal bed. While the fining-upward tidal flat parasequence is rare, it provides a caveat to the interpreter that geology is never simple. Boundaries. The parasequence boundary is a marine- flooding surface, which represents a relative rise in sea level (unlike the sequence boundary, which represents a rela- tive fall in sea level). If, however, my suggestion is correct that a parasequence can be viewed as a very high-order sequence in miniature, then there is a sequence boundary at its top and the transgressive systems tract is either very thin or absent. The marine flooding surface is then actu- ally the maximum flooding surface/condensed section. The opposing concept that an erosional event at the top of a parasequence is a product of the transgression echoes the old view about the nature of transgressive sheet sands. Van Wagoner et al. devote considerable discussion to the interpretation of transgressive lag deposits and observe that few true transgressive lag deposits have been observed on 1374 THE LEADING EDGE OCTOBER 1998 OCTOBER 1998 THE LEADING EDGE 0000 The parasequence J.W. MULHOLLAND, Littleton, Colorado Editor’s note: The Geologic Column, a monthly feature in The Leading Edge, is (1) produced cooperatively by the SEG Interpretation Committee and the AAPG Geophysical Integration Committee, and(2) coordinated by M. Ray Thomasson and Lee Lawyer. This article is the third in a series on sequence straigraphy by J.W. Mulholland. The previous arti- cles appeared in January 1998 and June 1998. IICOLUMN GEOLOGICI
description
Examine all the data in a reconnaissance fashion before starting to pick horizons. If a 3-D seismic cube is available, scroll through it in various orientations (timeslices, arbitrary lines, etc.) to help defi ne the important structural and stratigraphic elements that could affect the interpretation.
Transcript of Mulholland 1998 - Parasequences
The parasequence is one of the fundamental stratal unitsin sequence stratigraphy. Although individual parase-quences are generally below the resolution of seismic lines,sets of stacked parasequence are distinguishable by theircharacteristic stratal patterns. Geologists working withwell logs base much of their interpretation on the recog-nition of individual parasequences, their vertical and lat-eral facies relationships, and their stacking patterns. Thusthe parasequence is of prime importance in recognizingsequences and interpreting depositional and tectonic his-tory from sequence development.
The term parasequence itself is a product of the devel-opment of the principles of sequence stratigraphy.Repetitive, shallowing-upward stratal units were usuallyknown as cycles, especially in the field of carbonate faciesgeology. The word cycle implies a repetitive time series,and seemed inappropriate for repetitive rock units. VanWagoner introduced the term parasequence in 1985 at theSEPM midyear meeting and later wrote: “This usage pre-served the dictionary use of the word ‘cycle’ by Vail et al.(1977) to indicate a time in which a regularly repeatedevent occurs and emphasized the relationship betweenthe parasequence and the sequence.”
It was the development of the concepts of parasequencestacking patterns as building blocks of sequences thatmade them all-important. The fact that parasequencesstack in orderly and predictable patterns controlled by rel-ative sea level enhanced prediction of depositional envi-ronments and made subsurface correlation of time andfacies more reliable. Improved correlations produced bet-ter reservoir models, new exploration plays, and moresuccess for the upstream oil and gas industry.
In a previous article, I discussed the architecture ofsequences, or the manner in which parasequences com-bine in predictable patterns to form sequences. This arti-cle will look in more detail at the parasequence itself, theoperative developmental processes, the vertical and lateralfacies relationships that result, and the depositional envi-ronments represented. The best guide to this informationis still AAPG Methods in Exploration Series, No. 7,Siliciclastic Sequence Stratigraphy in Well Logs, Cores, andOutcrops, by J. C. Van Wagoner, R.M. Mitchum, K.M.Campion, and V.D. Rahmanian. This report will be mostlya synopsis of this excellent 1990 publication. The three fig-ures in this article also come from that source.
Characteristics. Parasequences have been identified in allcoastal environments where sea-level variation and sedi-ment variability are sufficient to produce recognizably dis-tinct facies. Most parasequences are progradational, andall exhibit shoaling-upward features. A siliciclastic parase-quence is essentially a miniature highstand systems tractterminated by a marine flooding surface (abrupt increase
in water depth). While a systems tract is composed ofparasequence sets, the parasequence is composed of bedsets.
It might be argued that if the upper boundary (theflooding surface and the thin strata just below it) wereexamined closely enough, it would have all the charac-teristics of a sequence boundary, complete with uncon-formity, lowstand, or shelf margin systems tract, and atransgressive systems tract. The flooding surface in effectis thus a “maximum flooding surface” and the shaledeposited atop the parasequence is a miniature condensedsection. In certain situations the erosional surface associ-ated with the marine flooding surface (usually in the prox-imal region of the parasequence) has been called atransgressive surface of erosion. Further detailed outcropstudies will be required to resolve the true nature of thissurface, but the fractal nature of sequence architecturefavors the idea that this erosion is the result of sea-levelregression rather than transgression.
In a significant but little known 1974 paper (Texas Bureauof Economic Geology Circular 74-1), David Frazier pointedout the (now) obvious, that the sediments for parase-quences are delivered by rivers to the coast, and parase-quences build seaward and fill the basin from the shoretoward the center. Prior to this, most geologists who hadn’t carefully thought the problem through envisionedbasins as filling from the middle or from the middle towardthe shore. As a case in point, it was widely accepted thattransgressive sheet sands were deposited along the shore-line as it migrated landward with the transgressing sea.We now know that there is really no such thing as a trans-gressive sheet sand. It is actually a series of sand bodiesdeposited as a retrogradational parasequence set, nowcalled a transgressive systems tract.
While all parasequences record shoaling-upward con-ditions, they do not all necessarily display coarsening-upward sediments. Van Wagoner and colleagues illustratethree basic parasequence stratal patterns: the beach parase-quence (Figure 1) and the deltaic parasequence (Figure 2),which are similar and coarsen upward, and the tidal flatparasequence (Figure 3), which is fining-upward and com-monly terminates with a coal bed. While the fining-upwardtidal flat parasequence is rare, it provides a caveat to theinterpreter that geology is never simple.
Boundaries. The parasequence boundary is a marine-flooding surface, which represents a relative rise in sea level(unlike the sequence boundary, which represents a rela-tive fall in sea level). If, however, my suggestion is correctthat a parasequence can be viewed as a very high-ordersequence in miniature, then there is a sequence boundaryat its top and the transgressive systems tract is either verythin or absent. The marine flooding surface is then actu-ally the maximum flooding surface/condensed section.The opposing concept that an erosional event at the top ofa parasequence is a product of the transgression echoesthe old view about the nature of transgressive sheet sands.Van Wagoner et al. devote considerable discussion to theinterpretation of transgressive lag deposits and observe thatfew true transgressive lag deposits have been observed on
1374 THE LEADING EDGE OCTOBER 1998 OCTOBER 1998 THE LEADING EDGE 0000
The parasequenceJ.W. MULHOLLAND, Littleton, Colorado
Editor’s note: The Geologic Column, a monthly feature in The LeadingEdge, is (1) produced cooperatively by the SEG Interpretation Committeeand the AAPG Geophysical Integration Committee, and(2) coordinatedby M. Ray Thomasson and Lee Lawyer. This article is the third in aseries on sequence straigraphy by J.W. Mulholland. The previous arti-cles appeared in January 1998 and June 1998.
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marine flooding surfaces. Clearly, this is a topic for fur-ther research. Van Wagoner et al. note that the floodingsurface is represented by correlative surfaces both land-ward and seaward of the coastal marine environment.Landward, it passes into the coastal plain and may beidentified by subaerial exposure, erosion, and fluvial depo-sition, none of which are readily traceable. On the marineshelf the correlative surface lacks evidence of erosion andis represented by pelagic and hemipelagic deposits.
In many situations a sequence boundary may coincidewith a parasequence boundary. For example, if a sequenceboundary includes incised valleys, the time-stratigraphicsurface it represents also exists between the valleys. Asthere may be no detectable erosion between the valleys,the surface most likely lies atop a parasequence and is coin-cident with the parasequence boundary. As parasequencesare commonly topped with sandstone beds that tend to bemore resistant to erosion, it follows that on the interfluveareas between valleys the sequence boundary will lie atthe top of the intervening parasequence.
The parasequence boundary, being a time-stratigraphicsurface, is an excellent horizon for correlation purposes.It is generally of relatively local extent, however, and bythe nature of sequence architecture is likely to display asigmoid downlapping pattern if traced regionally. It isthese surfaces that produce many of the reflections seis-mic stratigraphers recognize in marine sections.Stratigraphers working with well log cross-sections willuse them to define the general geometry of stratigraphicsections prior to working out the details of the fluvial/estu-arine sediment packages (i.e., the reservoirs).
Vertical facies relationships. Facies relationships are theprovenance of sedimentologists, and the details are notappropriate for this overview. As stated above and in a pre-vious article, parasequences display shoaling-upward char-acteristics. For siliciclastics this generally means faciestransitions from deeper-water marine shales through bio-turbated sand-shale interbeds with hummocky bedding ofthe lower shoreface, to massive trough cross-bedded uppershoreface sands, capped by planar laminated beach sands,
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WITHIN EACH PARASEQUENCE:Sandstone beds or bed sets thicken upwardSandstone/mudstone ratio increases upwardGrain size increases upwardLaminae geometry become steeper upwardBioturbation increases upward to the parasequence boundaryFacies within each parasequence shoal upwardParasequence boundary marked by:
• Abrupt change in lithology from sandstone below to mudstone above• Abrupt decrease in bed thickness• Possible slight truncation of underlying laminae• Horizon of bioturbation; burrowing intensity decreases downward• Glauconite, shell hash, phosphorite, or organic rich shale • Abrupt deepening in depositional environment across the boundary
Figure 2. Stratal characteristics of a deltaic parasequence.
OSMB = OUTER STEAM-MOUTH BAR
DF = DELTA FRONT,
PRO D = PRO DELTA
SH = SHELF
Figure 1. Stratal characteristics of a beach parasequence.
WITHIN EACH PARASEQUENCE:Sandstone bed sets and beds thicken upwardSandstone/mudstone ratio increases upwardGrain size increases upwardLaminae geometry become steeper upward (in general)Bioturbation decreases upward to the parasequence boundaryFacies within each parasequence shoal upwardParasequence boundary marked by:
• Abrupt change in lithology from sandstone below the boundary to mudstone or siltstoneabove the boundary
• Abrupt decrease in bed thickness• Possible minor truncation of underlying laminae• Horizon of bioturbation; bioturbation intensity diminishes downward• Glauconite, phosphorite, shell hash, organic rich shale, shale pebbles• Abrupt deepening in depositional environment across the boundary
estuarine tidal bars, and coal beds. For carbonates, thefacies (depending strongly on the environmental setting)are commonly represented by transitions from offshoremarine mudstones through nearshore skeletal wackestonesto ooid grainstones or algal stromatolites and tidal flats.
Lateral facies relationships. Walther’s law applies here,and the lateral facies transitions are the same as the verti-cal facies transitions. Parasequences are constructed ofbeds and bed sets, and the facies changes occur bed by bed.In the seaward direction, facies change from beach toshoreface to offshore marine shale. Parasequences termi-nate seaward by thinning, shaling out, and downlappingonto the sea floor. Parasequences terminate landward inone of three ways: (1) by onlap onto a sequence bound-ary; (2) by local or regional erosion due to coastal plainstreams or sequence boundary formation; and (3) by ter-mination of facies, such as a flood tidal delta, into the non-descript coastal plain complex.
Because a parasequence represents a fairly small timeslice and includes a full range of depositional environmentsfrom coastal plain to marine shelf, it makes a good map-ping unit. A geologist attempting to map a larger strati-graphic package will be including a number ofparasequences offset from one another according to theirstacking pattern, and both facies and isolith maps will besmeared representations of the actual paleogeography. Theconcept of the transgressive sheet sandstone discussedabove is an excellent historical example of this error.Effective paleogeographic mapping will confine itself to asingle parasequence. To properly map a complete sequence,the geologist must map the paleogeography parasequenceby parasequence, which will reveal the same depositionalpatterns translated across the shelf and back again as sealevel rises and falls through the transgressive and regres-sive systems tracts.
Processes. Parasequences form when the rate of genera-tion of accommodation exceeds the rate of sediment sup-ply to the coast. Because relative sea level is always in astate of change, accommodation is also. As accommoda-tion declines due to failing relative sea level or the risingsediment wedge, sedimentation patterns change fromaggradational to progradational. A relatively abrupt risein relative sea level (caused by sediment compaction inprodelta muds due to channel avulsion, tectonic subsi-dence, or eustasy) restores accommodation, generates aflooding surface terminating the parasequence, and thecycle repeats.
Parasequence sets. “A parasequence set is a succession ofgenetically related parasequences forming a distinctivestacking pattern bounded by major marine-flooding sur-faces and their correlative surfaces.” (Van Wagoner et al.)As discussed in a previous article on sequence architec-ture, parasequence sets are related to and named accord-ing to the state of relative sea level for the sequence theybelong to. Stacking patterns and hence parasequence setsmay be either aggradational, retrogradational, or progra-dational, and are known respectively as shelf margin, trans-gressive, and highstand systems tracts.
Summary. The parasequence is the fundamental unit inthe architecture of sequences of third, fourth, and fifthorder. Parasequences are progradational, shoaling-upwardstratal units that occur in all shoreline environments. Mostare also coarsening-upward, with one noted exception.Parasequence boundaries are marine flooding surfaces,which are time-stratigraphic horizons. As such they makegood correlation surfaces both in well log cross-sectionsand in seismic sections.
Seismic interpreters will see parasequences only asreflections, and the arrangement of those reflections willallow them to identify sequences. Geologists workingwith outcrops, cores, well logs, and cross-sections willimmediately recognize them as shoaling and/or coarsen-ing-upward stratal units. It will be important for them tobe able to recognize where they fit in a sequence modeland to understand what happens to them laterally in orderto make correct correlations. The parasequence is the keyto valid geologic mapping and it is the link between sed-imentology and stratigraphy. LE
Corresponding author: J. W. Mulholland, 7725 West Walker Drive,Littleton, CO 80213.
1376 THE LEADING EDGE OCTOBER 1998 OCTOBER 1998 THE LEADING EDGE 0000
WITHIN EACH PARASEQUENCE:Sandstone beds or bed sets thin upwardSandstone/mudstone ratio decreases upwardGrain size decreases upwardBioturbation increases upward to the parasequence boundaryParasequence boundary marked by:
• Abrupt change in lithology from mudstone or coal below the boundary to sandstone abovethe boundary
• Abrupt increase in bed thickness• Truncation (several 10s of feet or less) of underlying strata• Abrupt deepening in depositional environment across the boundary
Figure 3. Stratal characteristics of a tidal flat para-sequence.
SBT = SUBTIDAL
INT = INTERTIDAL
SRT = SUPRATIDAL
