"Tectonic Cycles of the North American Craton"
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For many years it has been recognized that the stratigraphic record contains considerable geologic evidence of both tectonic and global sea-level change. Once geologists began to look in detail at the distribution, relative age, and continuity of various strata units, they began to establish a series of large-scale stratigraphic disconformities that recorded periods of time when the land-surface was exposed and erosion ocurred.
In 1963, Larry Sloss, having studied the spatio-temporal distribution of rocks on the North American Craton, realized that the deposition of rocks on the craton recorded periods of relative sea-level highs while the development of unconformities and truncation of strata represented sea-level lows. Using this concept, Sloss (1963) delineated a series of "megasequences" which were deposited upon the craton and separated by widespread unconformities. These "megasequences," as illustrated to the right, showed evidence of stratal onlap onto the craton where strata show transgression or sea-level rise. Moreover, these same megasequences show evidence for an upward change to stratal offlap patterns associated with regression or sea-level lowering. Subsequent to sea-level drop, major regions of the craton were left exposed and unconformity ensued.
Although he did not realize the full implications of his observations at the time, Sloss's (1963) concept of correlating unconformity-bounded sequences helped to later revolutionize the study of sedimentary geology. Nearly 15 years later, geologists at Exxon studied the spatial and temporal distribution of sedimentary materials through seismic stratigraphic methods and recognized that within Sloss's large "megasequences' were a variety of lenticular shaped sedimentary packages bounded by seismic reflectors. These were also interpreted as regional unconformities, but at a smaller scale. This work, first published by Vail and others (1977), highlighted the importance of these reflectors as important higher-order time-lines representing periods of exposure and erosion, or at the least periods of non-deposition, and called these horizons sequence boundaries. These researchers recognized that when correlated offshore these reflectors expanded into conformable successions having no signficant break in sedimentation. In Sloss's figure to the right, the small stepped notches superimposed on each of the megasequences represent the scale of sequences studied by the Exxon team. By delineating individual, unconformity-bounded sedimentary packages by their seismic profiles and comparing the seismic data to the rock-record, Vail and colleagues (1977) were able to create a series of models to explain the development of patterns they had observed.
Based on their studies, Vail and others (1977) established the basic sequence stratigraphic model that provided a system of description for the observed depositional geometries and their chronostratigraphic relationships. Their model also predicted how a given sedimentary package might accumulate patterns of sea-level change and subsidence (Vincent, et al., 1998).
Although, the sequence framework was initially developed for siliciclastic-dominated basins, the application of sequence stratigraphy to carbonate-dominated or mixed siliciclastic-carbonate depositional systems follows roughly the same conceptual model developed by Vail and colleagues (1977). The following discussion emphasizes the patterns associated with sequence stratigraphic models in the context of the Trenton Group. For an excellent description of sequence stratigraphy and its components refer to Dr. Steve Holland's website entitled "An Online Guide to Sequence Stratigraphy" as available at the following url: http://www.uga.edu/~strata/sequence/seqStrat.html. >>Back to Top
SEQUENCE STRATIGRAPHY: THE APPROACH
As was mentioned above, sequence stratigraphy is a descriptive tool used by stratigraphers to establish or predict the spatial patterns of deposition of a constrained sedimentary succession deposited during a single sea-level rise and fall cycle. Sequence stratigraphy is defined as a stratigraphic method that uses unconformities (sequence boundaries) and their correlative conformities to package sedimentary successions into spatially and temporally constrained sequences (Vail et al., 1991; Vincent et al., 1998; Emery & Myers, 1996). Any unconformity bounded "sequence" can then be divided internally into smaller-scale genetically related units (systems tracts) deposited during individual phases of sea-level change ( i.e. transgression, high-stand, regression, and low-stand). This method is a powerful tool in modern stratigraphic studies because it integrates many aspects of stratigraphy including seismic stratigraphy, lithostratigraphy, cyclostratigraphy, event-stratigraphy, and biostratigraphy into a single stratigraphic framework. The development of a sequence stratigraphic framework for any given depositional basin provides the stratigrapher not only with a temporal framework for studying depositional change, but it also provides a spatial framework.
The following discussion introduces the concept of different scales (temporal and spatial) of sequence development observed in the rock record of the Upper Ordovician. An important aspect of sequence stratigraphy is its use in basin analysis through the establishment of very low-resolution (megasequence scale) to very high-resolution depositional spatio-temporal patterns (parasequence scale). The ensuing discussion describes several orders of sequences observed within the Ordovician. The magnitude of such sequences, as discussed by a number of authors including Van Wagnoner (1988), and Vail and colleagues (1991), generally refer to a variety of scales of temporally and spatially constrained depositional sequences. Those deposited during long time scales (i.e. 80-90 million years) and with large sedimentary thicknesses (1000's of meters) (megasequences of Sloss) are generally referred to as 1st-order sequences. Internally within these megasequences a number of 2nd, 3rd, 4th, and 5th-order sequences are developed over shorter time-scales, and range from 100's of meters down to meter-scale thicknesses in the case of higher-order depositional sequences. Low-order sequences are generally considered to be composite sequences and are composed of a hierarchy of smaller-scale, higher-order depositional sequences and parasequences. >>Back to Top
ORDOVICIAN TIPPECANOE MEGASEQUENCE
It is clear from the sedimentary record on the North American craton, that Sloss's Tippecanoe megasequence represents one of the highest sea-levels ever recorded in the Phanerozoic. Ranging in duration from the base of the Middle Ordovician to the end of the Lower Devonian, the development of the Tippecanoe megasequence represents one of the longest periods of sea-level highstands at between 80 to 90 million years. The Tippecanoe megasequence was not deposited by a single large-scale, long-term sea-level rise and fall event. Instead, it was punctuated by the development of a series of additional unconformities effective on a variety of shorter and narrow temporal and spatial scales.
One of the most important regional unconformities, was produced as a result of tectonism and the end Ordovician Glaciation. This unconformity, referred to as the Cherokee Unconformity, separates the Tippeacanoe Megasequence into an earlier phase (Creek Holostrome; after Wheeler, 1963) and a later phase (Tutelo Holostrome; after Wheeler, 1963), with each approximately 40-45 million years in duration. The Cherokee Unconformity is correlated across most of North America, and coincides with the Ordovician-Silurian boundary.
In addition to the major delineation of the Tippecanoe into its lower Creek and upper Tutelo phases (2nd-order? sequences), the diagram above also shows a sea-level curve by Greenlee and Lehmann (1993). The diagram illustrates the high-frequency of short-term, low magnitude sea-level oscillations during both holostrome phases as well as during the overall Tippecanoe megasequence. These high-frequency sea-level changes represent much shorter duration events, that when considered collectively produce the patterns associated with the larger-scale megasequences. For the purposes of this discussion, the development of these high-frequency sea-level changes is directly related to changes seen in the deposition of the Trenton Limestone during the Late Ordovician. As such, the remainder of this discussion will be focused on sea-level changes and sequence stratigraphic interpretations for the upper Mohawkian strata of the eastern U.S. including New York State. >>Back to Top
Image Modified after Sloss, 1963
"Tectonic Cycles of the North American Craton"
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3rd-ORDER SEQUENCES IN THE MOHAWKIAN
Beginning in the mid-to-late 1990's, Steve Holland (University of Georgia) and Mark Patzkowsky (Pennsylvania State University) published a series of papers investigating the sequence stratigraphy of Mohawkian and Cincinnatian strata. These studies by Holland and Patzkowsky (1996, 1997, 1998) helped to establish six large-scale third-order sequences (M1-M6) for the Mohawkian aged strata of the Nashville Dome to Cincinnati Arch Region with an additional six sequences (C1-C6) in the overlying Cincinnatian. Overall, the M1-M6 sequences each have a duration between 1 to 2 million years, and reflect deposition in the base of the Tippecanoe Megasequence (Creek Phase) as defined by Sloss (1963) and Wheeler (1963).
The figure to the right shows the chronostratigraphic assessment of Holland and Patzkowsky's sequences relative to the Late Ordovician Mohawkian and Cincinnatian Series and their associated stages. Note that the upper portion of the Mohawkian, referred to as the Chatfieldian Stage (sensu Leslie and Bergström, 1996), is dissected into two main sequences, the M5 and M6 sequences for both the Nashville Dome and Cincinnati Arch regions. Given the absolute time-scale estimates provided, Holland & Patzkowsky's sequences equate roughly to third-order sequences of 1 to 2 million years each. Based on the extrapolated absolute age dates, it appears that the duration of the late Mohawkian was approximately 3 to 4 million years during the course of which the Trenton Limestones were deposited. >>Back to Top
SEQUENCE STRATIGRAPHY OF THE TRENTON GROUP
Although the sequence stratigraphic frameworks constructed by Holland and Patzkowsky provide an excellent assessment of the stratigraphic record in the southern and central Appalachian Basin, their studies were not directly related to the type Mohawkian strata of New York State. Recent studies by Cornell (2001), Cornell and Brett, 2002, on the lower Trenton Group and the underlying Black River Group, and Brett and colleagues (2004) have investigated the New York succession in order to construct a similar sequence stratigraphic framework. Using a number of different correlation criteria, these workers have constructed a sequence stratigraphic framework based on the studies of Holland and Patzkowsky. The following figure is adapted from Brett et al., 2004, to illustrate the correlation of lithostratigraphic units, and sequences between New York State and the Jessamine Dome of central to northern Kentucky.
As shown, the overall sequence framework of Mohawkian sequences (M4-M6 of Holland and Patzkowsky) have been identified in the New York and central Kentucky regions. Although the three sequences of Holland and Patzkowsky are recognized, Brett and colleagues (2004) recognized that both of the third-order (M5 and M6) sequences can be further sub-divided into a series of smaller-scale, unconformity-bounded high-order sequences and sequence components. The high-resolution sequence framework of Brett and colleagues (2004), have defined six high-order sequences (fourth-order?), each representing between 400,000 to 500,000 years as extrapolated from the absolute ages established by Holland and Patzkowsky. >>Back to Top
As mentioned, several orders of cyclicity are recognizable in the Upper Ordovician mixed carbonate-siliciclastic strata of eastern North America. The smallest correlatable cycles are meter-scale shale-limestone successions, interpreted as parasequences (sensu Van Wagoner et al., 1988). Within the context of the Trenton Limestone, Brett and Baird (2002) have explained a number of these meter-scale cycles as presented in the discussion on lithostratigraphy. In subtidal shelf facies these cycles commence with thin-bedded calcisiltites/lutites and shales and pass upward into bioturbated nodular to wavy-bedded wacke- and packstones and finally into amalgamated pack- and grainstones (Brett and Baird, 2002).
Larger discontinuity bounded depositional sequences of at least two orders of magnitude are also recognizable within the Trenton Limestone. Decameter-scale sequencesare comparable to larger sequences and include thin analogs of transgressive and highstand systems tracts. These sequences are typically 5 to 15 m thick, and are thought to record depositional cycles of a few hundred thousand years, comparable to fourth- order cycles of Vail et al. (1991).
Larger scale sequences have thicknesses of tens of meters and inferred durations of between 1 to 2 million years, falling within the envelope of third-order sequences (Vail et al., 1991). Sequences recognized by Brett and colleagues (2004) represent subdivisions of the third-order composite sequences (M5 and M6) as recognized by Holland and Patzkowsky (1996, 1998). >>Back to Top
Image Modified after Holland and Patzkoskwy; 1996, 1998
Image Modified after Brett et al., 2004
"Comparative sequence stratigraphy of two classic Upper Ordovician successions, Trenton Shelf (New YorkOntario) and Lexington Platform (KentuckyOhio): implications for
The diagram to the right shows a composite stratigraphic column for the entire Trenton Group from the Trenton Falls to Black River Valley region of New York State. Important stratigraphic marker beds discussed in other sections of this website are labeled to guide the reader. The roll-over image shows the sequence stratigraphic interpretations based on the assessments of Brett and colleagues (2004). The right hand side shows the relative pattern of sea-level rise (overall deepening-upward) as indicated by blue triangles, and sea-level fall (overall shallowing-upward) as indicated by yellow triangles. The image indicates the relative position of key sequence stratigraphic intervals and surfaces including: transgressive systems tracts (TST's), highstand systems tracts (HST's), regressive systems tracts (RST's), sequence boundaries (SB), maximum flooding surfaces (MFS), and forced regression surfaces (FRS). In the diagram the sequence components are color coded with TST's shaded in light green, HST's shaded in pale yellow, and RST's labeled in pink. The following discussion presents a summary of descriptions for each individual sequence illustrated in the diagram to the right.
The basal Trenton sequence begins with the Watertown Limestone
which was previously
In the central Mohawk Valley, the shaly calcisiltites of the Napanee Formation, overlie the Selby-Watertown interval with unconformity (Cameron and Mangion, 1977). This sharp corrosion surface was formerly interpreted as a sequence boundary separating the Black River and Trenton Groups. Cornell (2000) has reinterpreted this contact as a submarine corrosion surface, as it displays evidence for dissolution, pyritization, and records a very high gamma-ray signature. Thus, this surface is now considered to be the maximum flooding surface of the M5A sequence.
The highstand interval of the M5A sequence, is represented
by the Napanee Formation of New York -Ontario. This interval is dominated
by rhythmically interbedded calcilutite/calcisiltite and dark shale facies,
and commonly shows a thick,
Near the very top of the Napanee, in several localities in the southern Black River to eastern Mohawk River Valleys (where the Napanee is still present), there is evidence of a sharp discontinuity surface and development of a few coarser-grained skeletal packestones. These are sharply overlain by a thick succession of very coarse-grained calcarenites of the Kings Falls Formation. This thin interval shows substantial shallowing and is inferred to represent a very thin RST.
On the Trenton shelf, the Rocklandian rhythmite facies (M5A HST) is succeeded rather abruptly by skeletal grainstone facies of the Kings Falls (upper Bobcaygeon or Kirkfield in Ontario). This sharp facies dislocation marks the M5B sequence boundary. and the surface shows substantial erosion in a number of localities in central New York, where the basal Kings Falls contains clasts of Black River lithologies, as well as Grenville basement rocks. In several localities in the Middleville area, the entire Napanee Formation is truncated by the basal M5B sequence boundary. The accentuation of this sequence boundary in some localities is directly related to local tectonically uplifted highs developed during the later part of the M5A sequence and in the lowstand of the subsequent M5B sequence.
In central to northern New York, the Kings Falls formation shows evidence for rapid upward transgression with very coarse grained facies grading upward into more condensed shaly nodular brachiopod, and echinoderm packestones and wackestones. The Kings Falls represents both the early and later condensed portions of the TST. In Ontario, the upper portion of this unit is famous for the development of the "Kirkfieldian Echinoderm Faunas."
The top of the M5B TST is slightly more obscure due to the lack of outcrop exposures. However, in the southern Black River Valley region, the top of the Kings Falls shows evidence for condensation and phosphatization of skeletal grains before transitioning into shaly nodular wacke- to packstones with abundant Prasopora seen in much of central New York. The maximum flooding surface of the M5B sequence is placed at the contact of the Kings Falls and Sugar River. In outcrop exposures of the type Kings Falls Formation this contact is just below a distinctive irregularly-bedded disturbed zone within the overlying Sugar River
The Sugar River Formation is composed of wavy bedded pack- to fine-grained grainstone facies particularly noted for beds containing the domal bryozoan Prasopora simulatrix, and the trilobite Cryptolithus tesselatus. This lithofacies represents deposition in substantially deeper water setting than much of the underlying Kings Falls and is interpreted as the HST of this sequence. Throughout much of the western Mohawk Valley to the northern New York region, the Lower Sugar River Formation is generally aggradational in character, but shows some minor evidence for progradational cycle development. The highstand facies of the Sugar River and correlative units in eastern New York show an abrupt upward change to dark Flat Creek Shales (Mitchell et al., 1994; Joy et al., 2000). In the eastern localities the contact with the Flat Creek shows evidence of extreme starvation, phosphatic-pyritic staining and is inferred to be the result of tectonically enhanced deepening associated with the migration of the Taconic Foreland Basin into eastern New York.
Although much of the lower to middle Sugar River is poorly constrained in outcrop, the transition into the later HST is associated with increased bioturbation near the contact with the Rathbun Member of the Sugar River. This interval, although dominantly fine-grained, is distinctive in the lack of substantial shaly interbeds found both below and above. In the Middleville, New York region this interval shows great abundances of Prasopora along a single bedding plane and represent an epibole horizon. >>Back to Top
Contrary to lower sequences, abrupt lateral changes are
seen in sequence M5C beginning in the Rathbun Member of the upper Sugar
River Formation. The lateral differentation of facies continues throughout
the remainder of the Denley Formation and higher sequences on the Trenton
Shelf. Although the basal sequence boundary of the M5C sequence is subtle
in most regions of the Trenton Shelf, the dramatic change to crinoidal grainstones in the West Canada Creek Valley stand in sharp contrast to
the underlying shaly-nodular fine-grained carbonates of the middle Sugar
River. As these coarser grained carbonates are traced both east
and west of Middleville, the facies transition laterally into finer-grained
carbonates that are difficult to separate from the remainder of the Sugar
River. It appears that the localized development of the Rathbun grainstones
is related to tectonic warping of the Trenton Shelf and development of
the "Middleville Arch." However, the expression of these grainstones
shows an upward deepening, upward-condensing pattern and is
interpreted as a TST succession.
The remainder of the Poland Member (above the basal Glendale sub-member) shows an upward coarsening series of small-scale cycles, which although thin, demonstrate a progradational pattern. This is also supported by faunal assemblage DCA scores as calculated by Gildner (2003) and shown in the section on biostratigraphy in this website.
Within the upper part of the Poland a series of amalgamated packstone and fine-grainstones represent a change to rapid shallowing and is thus interpreted as the transition out of the lower HST and into the RST interval.
Again as in the underlying sequence, the sequence boundary is generally subtle and recognizable through the identification of the overlying condensed zone of the overlying sequence. >>Back to Top
Brett and colleagues (2004) recognize the transition out of the upper Poland Member of the Denley into the Russia Member as representing a very condensed, basal transgressive systems tract. Although very fine-grained and calcilutitic, the development of very pure carbonates with very little clastic component represents a period of siliciclastic sediment starvation. In this case, the capping beds of the Poland are afossiliferous and their cap is accentuated by the deposition of the twin Kuyahoora K-bentonites. Unlike the equivalent sequence in Kentucky, which was identified by Holland and Patzkowsky (1996, 1998) by a sharp karstic contact within the Perryville Member and recognized as a major sequence boundary, the same sequence boundary in New York is very subtle and is developed in substantially deeper water facies.
The remainder of the TST interval shows several upward-deepening, retrogradational cycles. Near the top of the TST of this M6A sequence condensation occurrs again with the development of several amalgamated condensed beds in the condensed late TST. These two condensed beds informally referred to as the "overhanging ledge bed" and the "Castle Road bed" most likely represent the condensed zone below the maximum flooding surface.
As in Kentucky, the change from the TST, into the overlying HST is well-defined by the dramatic development of thin-bedded calcilutites and shales of the middle to upper Russia Member, thus representing a widespread deepening. This facies is widespread and analogous to that seen in the basal Trenton M5A sequence in the Napanee Formation. The occurrence of K-bentonites, widespread soft sediment deformation or "seismites" within the later part of the HST (in the Upper High Falls submember of the Russia), and dramatically increased siliciclastic input on the Trenton Shelf signals intensified tectonism during this time. Based on correlations of the Denley Formation downramp into siliciclastic dominated deposits, much of this tectonic development resulted in substantially steepened ramps during this time and is closely timed with the Amorphognathus tvaerensis / A. superbus conodont zonal boundary discussed in the biostratigraphy section of this website.
The upper part of the M6A sequence is very well developed. The later part of the HST (going into the RST) is represented in Trenton Falls, by the transition out of the Upper High Falls submember of the Russia, into the basal Rust Formation. The development of two unique cycles represented by the Taylor Fork Bed and the "Lower Disturbed Zone," signals a dramatic change to coarser grained limestones and shallower depositional conditions. This interval shows evidence for progradation and most likely represents the RST interval. >>Back to Top
The sequence boundary of sequence M6B is fairly sharp near the base of the Mill Dam Member of the Rust Formation. Although evidence for erosion is minimal, the sequence boundary is established at the upper contact of the "Lower Disturbed Zone" where the shaly nodular to brecciated fabrics of the disturbed zone transition to more massively-bedded coarse grained packstones and grainstones of the Upper Mill Dam.
Within the Mill Dam the coarse-grained brachiopod and echinoderm
facies show only minor argillaceous interbeds and cycles are poorly
constrained. However, the taphonomic signature of large Rafinesquina brachiopods as well as other fossil fragments show an upward change from abraded and
fragmented textures to substantially more complete and pristine fossil specimens.
This change in taphonomic signature, along with an increase in mineral
staining of bedding caps suggests that this interval is indeed deepening
and condensing-upward. As such, the Mill Dam Member is thus interpreted
as a TST interval.
The upper surface of the Mill Dam TST in some localities in the Mohawk Valley is marked by pyrite and phosphate impregnated hardgrounds showing evidence for corrosion. Thus the contact with the overlying Spillway Member is interpreted as a maximum flooding surface.
Overlying the MFS, as mentioned above, is the Spillway Member. On one scale of observation, the Spillway Member represents a single shallowing upward parasequence, it is composed of several smaller-scale cycles that become amalgamated upwards through the loss of shaly interbeds. As is common in most HST deposits, cycles show a general upward shallowing, progradational stacking pattern.
In the case of the Spillway Member, the late HST although very thin, shows very dramatic evidence for channelization and slumping associated with rapid sea-level fall. Within the succession at Trenton Falls, the base of the spillway for the hydrodam is composed of the beds of the "upper disturbed zone" and demonstrate the dramatically folded and convoluted strata at the top of the Spillway Member. Thus defined, the "Upper Disturbed Zone" represents the regressive systems tract of the M6B sequence. >>Back to Top
The M6C succession in the Trenton Falls region begins with a very thin zone of grainstones at the base of the Prospect Quarry member sharply overlying and infilling structural depressions formed on the top of the Spillway deformed interval. Both the top and bottom contacts of the Spillway deformed interval represent depositional discontinuities, which in the case of the upper contact is interpreted as a sequence boundary.
As mentioned, the thin grainstone interval deposited ontop of the underlying Spillway member shows dramatic evidence for upward condensation, and is capped by a pebbly phosphatic intraclast bed that is is sharply overlain by thin-bedded, shaly calcilutite carbonates of the Rust Quarry submember. This thin coarse-grained limestone package to shaly interbedded calcilutite interval is interpreted as the TST, MFS, early HST of the M6C sequence.
Of particular importance is the unique preservation pattern and depositional history of the early HST of this sequence. As described by Brett and colleagues (1998), the basal portion of the Prospect Quarry Member is represented by the Walcott-Rust Quarry interval, from which many well-preserved species of trilobites, echinoderms and other faunas have been collected. The exquisite preservation of these fossils indicates relatively deep water settings and are in stark contrast to underlying and overlying grainstone facies. The HST interval of this sequence is fairly thin but shows, as with lower sequences, an upward shallowing pattern out of the fine-grained shaly lutite cycles of the Rust Quarry beds into bioturbated wackestone to packstones of the upper Prospect Quarry member.
The top of the Prospect Quarry Member of the Rust Formation is marked by the change upward into regressive fine-grained grainstones that show some evidence of deformation, especially in the equivalent sequences in Kentucky. >>Back to Top
In contrast to underlying sequences, dramatic changes in sedimentation and basin configuration appear to have occurred during deposition of the C1 sequence in New York. In the Trenton Falls to northern New York region a widespread interval of crinoidal grainstone, referred to as the Steuben Limestone signals relatively shallow, but transgressive conditions, and sharply overlies the underlying Rust Formation. With the development of cross-stratified crinoidal sand shoals, especially in the Trenton Falls region, the Steuben formation most likely represents the TST component of the C1 sequence.
The uppermost Stueben passes both upward and basinward into a succession of fine-grained turbiditic calcarenites-calcisiltites and interbedded black shales. The upper contact of the Steuben, where conformable, shows an abrupt transition to a back-stepping succession of shales and argillaceous packstones and then into a major shale-rich succession of the Indian Castle Shale of New York (Baird and Brett, 2002). Given these observations, the upper contact of the Steuben is interpreted as the MFS, with the overlying Hillier Formation representing the HST deposits of the sequence.
As the Steuben represents the culmination of the Trenton, further discussion of sequence architectures is deferred. >>Back to Top