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Home , 1988 in Michigan, Antrim Shale, Cabot Head Shale, Eau Claire Formation, Ellsworth Shale, Manitoulin Dolomite

Structural sequences and styles of subsidence in the basin

Paul D. Howell* Department of Geological Sciences, University of , 101 Slone Research Building, Lexington, Kentucky 40506-0053 Ben A. van der Pluijm Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, Michigan 48109-1063

ABSTRACT lower crustal attenuation during the narrow- rates increase eastward for sequence C, and all subsidence episodes. Recently proposed dy- three wells show significant subsidence for se- Subsidence in the produced namic topography related to initiation of quence D. ~5 km of sedimentation over a period of more provides a driving This irregular basin subsidence history may than 200 m.y. during Paleozoic time. Utilizing mechanism for long-wavelength eastward tilt- hold the key to resolving the origin of the Michi- well-log correlations and constrained by com- ing. Together with a temporal correlation to gan basin. Subsidence curves from other cra- paction corrections and estimates of paleo- Appalachian tectonism, these mechanisms tonic settings, such as the , Paris, and bathymetry, we recognize four different styles provide a plate tectonic framework for the his- basins (Heidlauf et al., 1986; Brunet of subsidence in the basin: trough-shaped, re- tory of the Michigan basin. and LePichon, 1982; Sclater and Christie, 1980), gional tilting, narrow basin-centered, and show rapid subsidence in their early history fol- broad basin-centered. Subsidence began as a INTRODUCTION lowed by a long period of gradually declining trough-shaped, northerly extension of the Illi- subsidence, which is typical of thermal contrac- nois basin during Late to Early Or- The Michigan basin is a nearly circular, in- tion following rapid lithospheric attenuation dovician time. This was followed by narrow, tracratonic basin 400 km in diameter and 5 km (e.g., McKenzie, 1978). In contrast, the Michi- basin-centered subsidence in Early to Middle deep with only minor structural disruption gan basin reveals a number of subsidence reacti- Ordovician time. Basin-centered subsidence (Fig. 1), yet it remains without a definitive ori- vations and cessations (Sleep and Sloss, 1978). ceased for ~30 m.y. during long-wavelength gin (Leighton, 1996). The geometry of the Subsidence rates, however, provide only a part (>1000 km) eastward tilting in Middle to Late Michigan basin has led to numerous proposals of the information. Additional evidence is derived Ordovician time, a pattern incompatible with for basin subsidence mechanisms, including from the style of subsidence in the basin. A cur- thermal-contraction subsidence models. Basin- thermal contraction following development of sory examination shows that the Michigan basin centered subsidence resumed in time, an isolated “hot spot” (Sleep, 1971; Nunn and has a very simple geometry; it is nearly circular in but with a broader distribution. A second epi- Sleep, 1984; Houseman and England, 1986), shape, has relatively smooth structural contours sode of narrow, basin-centered subsidence oc- metamorphic phase changes in the crust (Haxby on its sedimentary horizons, and has only minor curred in latest Silurian through Middle De- et al., 1976; Middleton, 1980; Ahern and syndepositional faulting that has an insignificant vonian time and was replaced by broad, Dikeou, 1989; Hamdani et al., 1991), litho- impact on the basin-scale geometry of strati- basin-centered subsidence at the end of Mid- spheric stretching (McKenzie, 1978; Klein and graphic units. In the most recent full compilation dle time. The geometry of Upper Hsui, 1987), free thermal convection (Nunn, of stratigraphic data, Fisher et al. (1988) con- Devonian and younger Paleozoic deposits sug- 1994), and intraplate mechanisms (DeRito cluded that continuous, basin-centered subsidence gests another eastward-tilting event, but re- et al., 1983; Lambeck, 1983; Howell and appears to be compatible with thermal contraction sults remain inconclusive due to erosion of van der Pluijm, 1990). Contention among these subsidence mechanisms, although they added that strata and uncertainties in their paleobathym- proposals centers mainly on irregular basin sub- some shifts in sediment depocenters require dif- etry. In addition to these subsidence patterns, sidence rates (Sleep and Sloss, 1978; Quinlan, ferent subsidence mechanisms. two distinct unconformity styles are present: 1987; Howell and van der Pluijm, 1990). In contrast, Howell and van der Pluijm (1990) basin-wide and marginal erosion. There is no Decompacted basement subsidence curves argued that the early history of the Michigan evidence for significant basin-centered uncon- for three representative wells from the western, basin included significant changes in basin formities as predicted by purely thermal central, and eastern portions of the basin docu- subsidence style (shape of the basin), which were mechanisms. A history of episodic subsidence ment the temporal and areal distribution of sub- accompanied by changes in subsidence rate reactivations is interpreted as the result of a sidence variability (Fig. 2). Sequence A dis- (Fig. 2) and a response of the depositional sys- stress-induced, crustal-weakening mechanism plays rapid subsidence in the western and tems to the pattern of subsidence. These changes for the narrow, basin-centered subsidence, central basin wells, but is barely present on the in subsidence style provide constraints on the whereas broad basin-centered subsidence is in- eastern margin. Sequence B has a pronounced possible subsidence mechanisms for the basin terpreted as thermal contraction related to subsidence reactivation in the basin center, but (cf. Coakley et al., 1994; Coakley and Gurnis, is completely absent along the eastern margin 1995). In addition, recent advances in crustal rhe- *E-mail: [email protected]. due to nondeposition and erosion. Subsidence ology and mantle dynamics reveal that these two

GSA Bulletin; July 1999; v. 111; no. 7; p. 974–991; 12 figures.

974 STRUCTURAL SEQUENCES AND STYLES OF SUBSIDENCE, MICHIGAN BASIN

Figure 1. Total thickness of Phanerozoic sedimentary rocks in the Michigan basin region (contours in kilometers). Area with fine grid represents positive gravity anomaly in central Michi- gan corrected for basin fill (after Haxby et al., 1976). Striped area is location of Keweenawan based on drilling and Bouguer gravity (after Hinze et al., 1975).

factors exert significant influence on the strati- STRUCTURAL SEQUENCES Wagoner, 1991), but they need not correspond graphic signature of regions through directly to any of these other sequence types. their roles in continental deformation (Brace and Howell and van der Pluijm (1990) grouped This definition of a structural sequence corre- Kohlstedt, 1980; Molnar, 1988), epeirogeny, and strata from the early history of the Michigan sponds more closely to the original working de- eustasy (Gurnis, 1992a, 1992b). basin into stratal sequences that were defined by finition of a sequence: “a sequence comprises an This paper presents a set of new correlations significant changes in basin-subsidence patterns. assemblage of strata exhibiting similar responses and interpretations for the stratal geometries of the This represents a different type of sedimentary to similar tectonic environments over wide areas, Michigan basin, building on data presented by sequence than the unconformity-bounded depo- separated by objective horizons without specific Howell and van der Pluijm (1990). These include sitional sequences of Mitchum (1977) and time significance” (paraphrased by Wheeler new data made available since previous compila- Van Wagoner et al. (1988) and the genetic se- [1958] from Sloss et al. [1949]). tions (Lilienthal, 1978; Fisher et al., 1988), new quences of Galloway (1989). A structural se- To a first-order approximation, isopach maps correlations made possible by improved well- quence is defined here as a succession of sedi- can be used to define structural sequences (Kay, logging techniques, and new stratigraphic group- mentary strata in a basin that is bounded by 1945), assuming that the changes in thickness of ings that differ from those used in previous stud- distinctive changes in the pattern of basin the strata under consideration are due to lateral ies (Fisher et al., 1988). The correlations are )subsidence. These sequences may correspond subsidence rate variations during deposition. presented in a framework of structural sequence to unconformity-bounded depositional se- Calvert (1974), however, recognized that isopach stratigraphy, which offers an alternative frame- quences, or to groups of depositional sequences maps record the structural deformation of a basin work with which to analyze basin evolution. termed composite sequences (Mitchum and Van between the time of deposition of the lowermost

Geological Society of America Bulletin, July 1999 975 HOWELL AND VAN DER PLUIJM and the uppermost strata of the interval examined only if certain reliability criteria are met. Com- paction of the interval of interest, syndepositional of the underlying units, and the paleobathymetries of both included and bound- ing strata must be considered.

METHODOLOGY

Petrophysical logs from more than 500 wells drilled in the Michigan basin for ex- ploration and subsurface waste disposal were correlated on the basis of lithostratigraphy (Howell, 1993). Isopach maps presented in this paper include the locations of wells used to con- strain the contours. New stratigraphic correla- tions have been obtained from this analysis and compared with those of Lilienthal (1978), Fisher et al. (1988), Catacosinos and Daniels (1991), Catacosinos et al. (1991), and Coakley et al. (1994). To determine the preliminary breakdown of strata into structural sequences, isopach maps from 43 stratigraphic intervals were examined in conjunction with well-log cross sections, litholo- gies, and core data. Possible depositional envi- ronments were considered for each unit and lithologic facies changes examined for support- ing evidence, leading to estimates of maximum and minimum probable paleobathymetries. In order to minimize possible effects of paleo- bathymetry on subsidence analysis, structural sequence boundaries were chosen only at inter- vals where the estimated paleobathymetry is small (i.e., relatively shallow water depths, Figure 2. Decompacted basement subsidence curves for three Michigan basin wells for the <30 m). Thin stratigraphic intervals were not first 120 m.y. of the basin’s history. Note that subsidence patterns differ across the basin for each considered for structural sequence status when stratigraphic interval identified as a structural sequence. Numeric ages follow Fisher et al. the uncertainty in paleobathymetry constituted a (1988) and Smith et al. (1993). The bottom curves (solid lines) represent the depth to basement significant proportion of the thickness of the through time after correcting for paleobathymetry (uppermost, dashed curves) and compaction. proposed sequence; uncertainty in paleobathy- Well numbers refer to listings in Howell (1993): #184—Amoco Schiller 1-10, Oceana County; metry of strata constitutes one of the important #74—Hunt-Martin 1-15, Gladwin County; #239—MichCon Osterland 1-14, St. Clair County. limitations of fine-scale stratigraphic resolution of geodynamic behavior (Calvert, 1974; Célérier, 1988). Intervals were also examined for evi- intervals were grouped informally into provi- ping” of the sedimentary load in the basin. Airy dence of gradational facies contacts, synsedi- sional structural sequences based on isopach assumptions (local isostatic balance) are probably mentary faulting, basinal extent of erosional un- geometries. Each structural sequence was exam- unrealistic in a small, flexural basin such as the conformities, and postdepositional dissolution of ined internally for consistency of the interpreted Michigan basin, where the basin margins are par- soluble lithologies, each of which could intro- subsidence geometry (e.g., basin-centered ver- tially depressed by the sediment loading in the duce uncertainty. Lithostratigraphic contacts de- sus wedge-shaped basin subsidence patterns). center of the basin (Haxby et al., 1976; Nunn and fined by time-transgressive facies changes pro- Basement subsidence was quantified by de- Sleep, 1984). Thus, local residual subsidence cal- duce stratigraphic surfaces that are typically compacting each structural sequence and the un- culations will overestimate tectonic subsidence on unacceptable as structural sequence boundaries; derlying sequences, and calculating the change in the basin margins and underestimate it in the isopach maps derived from stratigraphic units basement depth during deposition of each se- basin center. Lithology-dependent decompaction with facies contacts for boundaries introduce quence. This change in depth to basement (here- methods used in this analysis follow those of multiple uncertainties into a structural analysis, after referred to as ∆DB) is used to define the sub- Angevine et al. (1990). We note that uncertainty including both paleobathymetric variability and sidence for each structural sequence, rather than a in compaction parameters was found to have little diachroneity of the boundary. Isopach maps derivative of the ∆DB such as backstripped resid- effect on our analysis; sensitivity testing revealed were compared for significant changes in rela- ual subsidence. Local backstripping (Steckler and small differences in overall shape of the basin de- tive shape, which were weighed against the pos- Watts, 1978) essentially produces a similar shape spite a wide range of decompacted subsidence sible uncertainties listed here, and stratigraphic as ∆DB, but a smaller amplitude due to the “strip- rates (Howell, 1993).

976 Geological Society of America Bulletin, July 1999 STRUCTURAL SEQUENCES AND STYLES OF SUBSIDENCE, MICHIGAN BASIN

Figure 3. Stratigraphic column for the Michigan basin showing strati- graphic extent of structural se- quences, unconformities, and in- ferred subsidence patterns. Note that Sloss sequence boundaries do not correspond directly with structural sequence boundaries.

STRUCTURAL SEQUENCES IN THE central basin and on the basin margins, and in- present an isopach map, a ∆DB map, and two MICHIGAN BASIN cludes the temporal ranges of the structural se- cross sections, along with discussion concerning quences described herein. The subsidence pat- the internal consistency of these sequences. To Strata of the Michigan basin are dominated by terns labeled for each structural sequence are facilitate comparison of changes in basin subsid- limestone and dolomite; there are significant sili- inferred from the shapes of the ∆DB maps and ence patterns, each ∆DB map is normalized to its ciclastic and evaporite components. Figure 3 supporting evidence within the sequences. maximum thickness (100 * thickness/maximum summarizes the stratigraphic succession in the For each of the six structural sequences we mapped thickness).

Geological Society of America Bulletin, July 1999 977 HOWELL AND VAN DER PLUIJM

Figure 4. (A) Isopach map of sequence A, Upper Cambrian Mt. Simon to Lower Ordovician Oneota Formation. Isopach values are in meters for Figures 4–10. (B) Map of base- ment subsidence (∆DB) during deposition of Sequence A. Con- tours are normalized (100 basis) to facilitate comparison with other ∆DB maps for Figures 4–10. (C) Stratigraphic cross sections for the units composing sequence A, with section loca- tions marked on the isopach and ∆DB maps. Note that clo- sure within the central Michi- gan area is more pronounced on the ∆DB map than the isopach due to greater decompaction corrections in central Michigan (greater current burial depth).

Sequence A (Cambrian–Lower Ordovician) Simon and of the Eau Claire is a facies separates the Lower Ordovician Oneota Forma- contact; Catacosinos and Daniels, 1991). This is tion (Prairie du Chien Group) from the Trem- The oldest Phanerozoic rocks in the basin overlain by thin Dresbach (Galesville) sand- pealeau (Lilienthal, 1978) may be an ash bed; were deposited in a trough-shaped stones, which are also in apparent facies contact however, no core information is available to con- that was open to the south (sequence A, Upper with both the Eau Claire and the overlying Fran- strain its lithologic nature. Cambrian–Lower Ordovician, Fig. 4). These conia shales and carbonates. Log correlations The southward-thickening, trough-shaped rocks were deposited under relatively shallow suggest that the contact is gradational between character of the ∆DB map (Fig. 4B) for this in- water conditions (Catacosinos and Daniels, Franconia carbonates and the Trempealeau terval is strongly influenced by the thick Mt. Si- 1991); maximum water depths were probably Formation. These gradational contacts and facies mon (Fig. 4C). The Franconia– tens of meters. The basal Mt. Simon Sandstone is changes prohibit the use of these individual Dresbach–Eau Claire interval also thickens to a widespread Upper Cambrian unit that thickens formations as reliable indicators of basin subsid- the south, but less dramatically. The Trem- southward into the northern , as does ence change; boundaries for structural sequences pealeau and Oneota strata thicken very gradu- the of the overlying . should be as nearly synchronous as possible. The ally toward central Michigan and become The boundary between the sandstone of the Mt. strong, persistent gamma-ray log marker that sandier northward. The younger units also over-

978 Geological Society of America Bulletin, July 1999 STRUCTURAL SEQUENCES AND STYLES OF SUBSIDENCE, MICHIGAN BASIN

Figure 5. (A) Isopach map of sequence B, Lower Ordovician Shakopee to Middle Ordovician Glenwood. (B) Map of basement subsidence (∆DB) during deposi- tion of sequence B. (C) Strati- graphic cross sections for the units composing sequence B. Isopach and ∆DB maps both dis- play broad basin subsidence.

step the Mt. Simon to the east on the Findlay such as paleobathymetry), and (3) there is a dis- Group), an ~500-m-thick succession of thin- arch and to the north, indicating a gradual tinctive change in basin shape between the bedded, shaly dolomites and sandstones con- broadening of the basin. Overall, the basin has Oneota and the overlying Shakopee Formations. taining as much as 30% anhydrite as thin beds the appearance of a trough-shaped depression in the central basin (Foster formation of Fisher that is a northward extension of the Illinois Sequence B (Lower Middle Ordovician) and Barratt, 1985). This unit represents shal- basin (Sleep et al., 1980). Although the strati- low-subtidal, intertidal-mudflat, and evaporitic graphic units composing sequence A do not Sequence B (Early–Middle Ordovician, environments (Fisher and Barratt, 1985). The have a uniform style of trough-shaped thicken- Fig. 5) consists of three stratigraphic intervals, Shakopee Formation is overlain by the St. Peter ing, they are considered as a single structural se- and represents the beginning of significant Sandstone, an ~300-m-thick, quartzose sand- quence because (1) they appear to show a grad- basin-centered subsidence and separation of the stone in Michigan containing thin interbeds of ual change in basin shape, (2) they cannot be Michigan basin from the Illinois basin to the shaly dolomite. The upper portion of the St. Pe- distinguished as separate structural sequences south. The lowest unit is the Shakopee Forma- ter formation is highly bioturbated, indicating due to their thinness (relative to uncertainties tion (upper portion of the Prairie du Chien normal-marine, shallow-water conditions,

Geological Society of America Bulletin, July 1999 979 HOWELL AND VAN DER PLUIJM whereas the lower half of this sandstone has than that of the underlying Cambrian units (Fig. 2), for compaction and paleobathymetry reveals that very limited bioturbation, suggesting restricted indicating that there was a substantial change in there is little or no basin-centered subsidence (possibly hypersaline) environmental condi- basin-centered subsidence prior to the end of Sauk component during sequence C deposition (Fig. tions similar to those of the underlying deposition, and (2) even a substantial (100 m) drop 6B), and the carbonate depocenters are primarily Shakopee Formation. Cored intervals from nu- in sea level could, with total erosion of exposed shallow-water platforms, not structural basin merous wells in the central part of the basin strata, cause only about a 200 m differential in centers (Howell, 1993). The Cincinnatian units document the shallow-water nature of this unit, thickness from basin margin to center. On the basis are capped by a regional erosional unconformity which is a significant reservoir for natural gas of the evidence cited here, we consider it more that is well documented across the basin and (Fisher and Barratt, 1985; Harrison, 1987; plausible that uplift of the basin margin was coeval margins (Taconic disconformity of Wheeler, Barnes et al., 1992). The St. Peter sandstones with increased subsidence and sedimentation in 1963; Lilienthal, 1978; Howell, 1993). grade upward into shaly, burrowed dolomites the basin center. The resulting isolation of the and sandstones of the , a basin also has correspondence in the change from Sequence D (Lower Upper Silurian) thin (<30 m) unit that represents continued open-marine conditions to the more restricted fa- shallow-water deposition. cies of the Shakopee and lower St. Peter Sandstone Silurian units of sequence D record a pattern of This thick succession of shallow-water sand- in Michigan’s central basin. broad basin-centered subsidence (Fig. 7). The stones and carbonates accumulated in a narrow, is a thin (<40 m), basal Silu- rapidly subsiding depression (Fig. 5B), marking Sequence C (Middle Upper Ordovician) rian, shallow-water carbonate unit that fills topo- the onset of significant basin-centered subsidence graphic lows on the Taconic disconformity, and in Michigan. On the basin margins, however, and Basin-centered subsidence ceased in Middle contains local biohermal mounds in the southern throughout eastern North America, significant ero- Ordovician time, replaced by eastward tilting to- part of the state. The Manitoulin Dolomite is over- sion and dissolution of carbonates occurred during ward the Taconic margin of North America (se- lain by the , a thin (<50 m) ma- this time, forming the post-Sauk unconformity of quence C, Middle to Late Ordovician, Fig. 6), rine shale that is found throughout the basin. In the Sloss (1963). Examination of cores and well logs Regional eastward tilting dominated the entire southern part of the state, these units have a recip- in this study and others revealed little physical ev- eastern during this period (Howell rocal thickness variation where thin Cabot Head idence and no paleontological evidence of this un- and van der Pluijm, 1990; Coakley et al., 1994), shales overlie locally thickened Manitoulin bio- conformity within the deep central basin (Fisher and may have extended northward across much herms. The overlying Clinton Group is repre- and Barratt, 1985; Catacosinos and Daniels, 1991; of eastern Canada and the Hudson Bay basin sented by a very thin, shaly interval in the southern Smith et al., 1993), although some have suggested (Cook and Bally, 1975; Quinlan and Beaumont, part of the state, which thickens into a shallow- that it may be present between the Shakopee 1984; Beaumont et al., 1988). In Michigan, the water, limestone bank as much as 130 m thick on Formation and the St. Peter Sandstone, particu- Black River and Trenton Group carbonates thin the northern side of the basin, and a thin, deeper larly along the southeastern flank of the basin (Al- markedly to the north and west from their depo- water, mudstone and wackestone facies in the cen- gonquin arch: Harrison, 1987; Barnes et al., 1992; centers in the southeastern portion of the state. tral basin (Harrison, 1985). This basin paleobathy- Nadon and Smith, 1992). Because the surrounding These thickness changes, however, are due in metry was enhanced in Middle Silurian time with region lacks correlative facies to the thin-bedded part to a facies change from shallow-water development of an extensive Niagara Group car- Shakopee Formation and the basal, nonburrowed wackestones and packstones in the area of the de- bonate bank around the basin perimeter, and a sandstones of the St. Peter Sandstone, Fisher and pocenter to starved, deeper water mudstones and deeper water area in the central basin. Pinnacle Barratt (1985) suggested that the Michigan basin wackestones in the central basin (Fig. 6C; Wilson reefs were abundant on the slopes of the carbonate continued to subside during early Middle Ordovi- and Sengupta, 1985; Howell, 1993). Subsequent bank and help constrain basinal water depths to cian time while uplift and erosion occurred on the shales and shaly carbonates of the Utica Forma- 200–300 m (Leibold, 1992). basin margins. Uplift and erosion are well docu- tion and Cincinnatian strata (undifferentiated) The Niagaran paleobathymetric depression was mented on the Kankakee arch (southern Michigan, filled in the moderate paleobathymetry (<60 m filled by the end of A-2 carbonate (Salina Group) northern , and Illinois; Fig. 1) where water depths) that resulted from these facies deposition, as evidenced by the presence of wide- pre–St. Peter formation erosion locally eliminated changes; thus, the return to a basin-centered spread sabkha deposits at the top of this unit as much as 200 m of Lower Ordovician and Upper isopach pattern for the Utica and Cincinnatian (Leibold, 1992). Subsequently, the basin contin- Cambrian rocks, regions later filled in with St. Pe- units does not reflect a return to basin-centered ued to subside and fill with evaporites and thin car- ter Sandstone (Buschbach, 1964; Willman et al., subsidence, but rather the infilling of an existing bonates of the Salina Group during Late Silurian 1975), and to a lesser extent this pattern extends bathymetric low (Howell, 1993). time. Postdepositional dissolution of evaporites on into (Smith et al., 1993). The thickness This tilting episode was not evident in the pre- the southern basin flank produced an artificially pattern of the St. Peter formation is well con- vious collections of isopach maps (Fisher et al., thin isopach interval, which leads to a local under- strained across the northern basin by more than 1988) primarily because the St. Peter Sandstone, estimation of the original ∆DB (Fig. 7B) for this 100 wells, but the more limited data shown in Fig- with its decidedly basin-centered isopach pat- structural sequence. However, the overall pattern ure 5A represent the paucity of wells that com- tern, was included in the interval with these later of basin-centered subsidence remains evident, pletely penetrate the sequence B section. Ordovician carbonates (Tippecanoe I sequence with a much broader distribution of subsidence Although it may appear that a simple drop in of Fisher et al., 1988). Fisher et al. (1988) and than is present in sequence B (saucer shaped for eustatic sea level, such as that postulated for the Coakley et al. (1994) suggested that the succes- sequence D versus bowl shaped for sequence B). end of the Sauk sequence of Sloss (1963), could sion of thickness variations in this sequence is Thick units within this sequence that have shal- cause a basin-centered subsidence pattern, this is suggestive of basinal subsidence migrating east- low-water indicators near the top and bottom also not the case for structural sequence B. There are ward through time (Black River and Trenton car- display the same overall, saucer-shaped subsid- two reasons for this: (1) the accumulation rates of bonates), followed by a return to basin-centered ence pattern as the entire sequence (e.g., Salina B the Shakopee and St. Peter intervals are far greater subsidence (). However, correction unit, F-unit; Leibold, 1992).

980 Geological Society of America Bulletin, July 1999 STRUCTURAL SEQUENCES AND STYLES OF SUBSIDENCE, MICHIGAN BASIN

Figure 6. (A) Isopach map of sequence C, Middle Ordovician Black River to Upper Ordovi- cian Cincinnatian. (B) Map of basement subsidence (∆DB) during deposition of sequence C. (C) Stratigraphic cross sec- tions for the units composing se- quence C. Lack of closure in southeastern Michigan on ∆DB map suggests that subsidence due to compaction of underly- ing units in the central basin is the cause of isopach closure and eastward shift of the depocenter.

Sequence E (Uppermost Silurian–Middle with a few thin halite beds) grade upward into 1963) cannot be recognized on the basis of well Devonian) nonfossiliferous carbonates and quartzose sand- correlations in the central basin despite extensive stones of the Bois Blanc and coverage, although some evidence of discon- The Bass Island carbonates and evaporites rep- formations. These two units are distinct on the formable lithologic relationships has been re- resent a return to narrow (bowl shaped), basin- basin margins, but are interbedded and strati- ported from cores (Gardner, 1974). This relation- centered subsidence in latest Silurian time, which graphically inseparable in the deep central basin. ship of an extensive margin unconformity with continued into Middle Devonian time (sequence An extensive unconformity present on the basin no measurable erosion in the central basin is sim- E, Fig. 8). In the central basin, these restricted- margins near the Silurian-Devonian boundary ilar to that of the post-Sauk unconformity within marine deposits (to 40% anhydrite by thickness (Fig. 3, post-Tippecanoe unconformity of Sloss, sequence B (Fig. 3).

Geological Society of America Bulletin, July 1999 981 HOWELL AND VAN DER PLUIJM

Figure 7. (A) Isopach map of sequence D, Lower Silurian Man- itoulin to Upper Silurian Salina G unit. (B) Map of basement subsi- dence (∆DB) during deposition of sequence D. (C) Stratigraphic cross sections for the units com- posing sequence D. Isopach and ∆DB maps both display broad basin subsidence. The tightening of the contours on the southwest- ern flank of the basin reflects ex- tensive halite dissolution in the Salina A-2, B, and F units. Loca- tion of Niagara reef growth is ap- parent on the cross sections.

The Sylvania–Bois Blanc deposits grade up- half of the basin, and a facies change to open- Sequence F (Uppermost Middle Devonian) ward into fossiliferous limestones of the Amherst- marine wackestones and mudstones in the eastern berg Formation (lower River Group), portion of the central basin (Gardner, 1974). Al- The Middle Devonian car- which represents open-marine conditions with a though the paleobathymetry at the top of the bonates and shales record another episode of rich coral and brachiopod assemblage, but no Dundee Formation is uncertain, the paleorelief is basin-centered subsidence, but with a broad dis- clear paleobathymetric gradients (Gardner, 1974). small relative to the thickness of this structural se- tribution (sequence F, Fig. 9). The paleobathym- These carbonates are overlain by the Lucas quence (~50 m maximum water depth versus etry of the at the base of this sequence Formation, a succession of shallow, restricted- 962 m maximum measured thickness), and there- is uncertain, and this unit may belong to the un- marine and intertidal carbonates and evaporites. fore introduces no significant uncertainty in the derlying sequence E (narrow basin-centered The sequence is capped by the Dundee Forma- ∆DB calculations for this interval. The individual subsidence pattern); however, because it is a thin tion, a marine carbonate that has been extensively thickness patterns of the Sylvania–Bois Blanc, unit and is considered part of the Traverse studied for its role as a major oil reservoir in the Lucas, and Dundee intervals (Fig. 8C) are all Group, it is treated within sequence F. The Tra- basin. The Dundee consists of lagoonal and compatible with the overall narrow basin pattern verse Group carbonates are considered moder- sabkha carbonates and anhydrite in the western displayed in Figure 8B. ately shallow water, marine deposits throughout

982 Geological Society of America Bulletin, July 1999 STRUCTURAL SEQUENCES AND STYLES OF SUBSIDENCE, MICHIGAN BASIN

Figure 8. (A) Isopach map of sequence E, Upper Silurian Bass Island to Middle Devonian Dundee. (B) Map of basement subsidence (∆DB) during depo- sition of sequence E. (C) Strati- graphic cross sections for the units composing sequence E. Isopach and ∆DB maps both display narrow basin subsi- dence. Although halite dissolu- tion may be significant in the Lucas evaporites, all units dis- play similarly narrow, basin- centered isopach patterns.

the basin on the basis of their faunal content, the Traverse carbonates, even a modest paleo- Upper Devonian– Strata presence of local bioherms, and the very gradual bathymetry (e.g., ~50 m deeper in the basin cen- (Provisional Sequence G) nature of facies changes across the basin (Gardner, ter or eastern portion) would cause little change 1974; Lilienthal, 1978; Fisher et al., 1988). The in the overall basin subsidence pattern. Much Upper Devonian and Mississippian strata shale content of the Traverse Group increases deeper basinal paleobathymetry (~150 m) would are extensively eroded around the basin mar- eastward across the basin from ~20% shale in have been required for the broad pattern de- gin, but the uneroded remnant in central western Michigan to as much as 80% shale in scribed here to result from a narrow subsidence Michigan suggests a final eastward-tilting the eastern region (Gardner, 1974). This struc- pattern similar to that of the underlying se- event (provisional sequence G, Fig. 10). These tural sequence has the broadest (most saucer quence E, but such substantial paleobathyme- units do not completely qualify for structural shaped) distribution of the sequences described tries are commonly recognized by the presence sequence status because the shape of the ∆DB here. Although there are few data available to of basin margin clinoforms (Sarg, 1988), and are across the entire basin is unclear due to the constrain the paleobathymetry of the uppermost not present here. limited lateral extent of the strata and uncer-

Geological Society of America Bulletin, July 1999 983 HOWELL AND VAN DER PLUIJM

Figure 9. (A) Isopach map of sequence F, Middle Devonian Traverse Group, including the Bell Shale. (B) Map of basement subsidence (∆DB) during deposi- tion of sequence F. (C) Strati- graphic cross sections for the units composing sequence F. Isopach and ∆DB maps both dis- play broad basin subsidence.

tainty associated with the paleobathymetry of distal limit of a westward-prograding delta com- Shale and interval thickens some units. plex derived from the Alleghanian orogenic welt eastward (Fig. 10, B and C), suggesting another The basal units of this interval consist of the (Gutschick and Sandberg, 1991). An isopach eastward regional tilting event toward the Al- Antrim and Ellsworth Shales. The map of the combined Antrim, Ellsworth, and leghanian foreland. Basinwide distribution of the is very organic rich, with a highly radioactive Berea-Bedford interval displays a distribution overlying Michigan Group and gamma-ray log character, and is restricted to the that is thickest to the west and thinnest in the are too limited areally to de- eastern and central part of the basin. To the west, south-central portion of Michigan (Fig. 10A). lineate regional subsidence patterns and are not this unit grades into the , which This distribution is interpreted in terms of mod- included in the analysis. is less radioactive and thickens westward erate paleobathymetry at the end of Berea- (Fig. 10C). Both units grade upward into in- Bedford deposition (perhaps ~50–100 m deeper MODELS FOR BASIN SUBSIDENCE terbedded shales and fine sandstones of the Bed- in the south-central area) superposed on a pat- ford Shale and formations. tern of regional subsidence (Gutschick and There is no single mechanism that fully ex- These coarser detrital sediments represent the Sandberg, 1991). The overlying Coldwater plains the origin and subsequent evolution of the

984 Geological Society of America Bulletin, July 1999 STRUCTURAL SEQUENCES AND STYLES OF SUBSIDENCE, MICHIGAN BASIN

Figure 10. (A) Isopach map of lower portion of sequence G, Up- per Devonian Antrim Shale, Ellsworth Shale, , and Berea Sandstone. Thinning in basin center reflects deeper water conditions. (B) Isopach map of middle portion of se- quence G, Marshall Sandstone and Coldwater and Sunbury Shales. Marshall is a coarser fa- cies equivalent to a portion of the Coldwater, and does not include sandstones of the “Michigan stray.” Overall eastward tilting pattern is evidence. (C) East-west stratigraphic cross section for the units composing sequence G, with section locations marked on the isopach maps (dashed lines).

Michigan basin. Rather, changing subsidence The following is a summary of the subsidence gested as a scenario that explains the major fea- styles of the basin require several mechanisms to patterns of the Michigan basin. Sequence G: Up- tures of the basin. be active through the course of Paleozoic time, as per Devonian to Mississippian, eastward tilt- documented by profiles of normalized ∆DB for ing(?); sequence F: Middle Devonian, basin cen- A. Rift Development each sequence (Fig. 11). The profiles illustrate tered, broad; sequence E: Lower Devonian, distinct changes in subsidence style between se- basin centered, narrow; sequence D: Silurian, Subsidence of the Michigan basin began in quences and highlight the significant difference basin centered, broad; sequence C: Upper Or- Late Cambrian time with deposition of the Mt. Si- between narrow and broad basin-centered pat- dovician, eastward tilting; sequence B: Lower mon Sandstone in a trough-shaped pattern, with a terns. Normalization to the maximum thickness Ordovician, basin centered, narrow; sequence A: deeper depocenter to the southwest in northern along each profile facilitates comparison of pro- Cambrian, trough shaped. Illinois (Buschbach, 1964). This pattern suggests file shape and subsidence style. The following sequence of mechanisms is sug- that the eastern midcontinent region underwent a

Geological Society of America Bulletin, July 1999 985 HOWELL AND VAN DER PLUIJM

Figure 11. Summary profiles of ∆DB for sequences A–G (ori- entations northeast-southwest except as indicated), with style of subsidence labeled. Portions of profiles within dotted lines em- phasize areas of basin with nor- malized ∆DB > 70%.

change in locus of extension from the Middle sion suggested for the area at that time. This struc- gin in the middle crust for this anomaly, and we Cambrian, east-west–oriented Rome trough arm tural sequence thus contains a gradual change in join these authors as we fail to conjecture as to the of the Reelfoot rift in Kentucky (McGuire and the basin subsidence pattern, one that is consistent origin of this feature. Howell, 1963; Cable and Beardsley, 1984) to the with an origin by lithospheric stretching. more north-south orientation of the Illinois- The rifting discussed here is distinct in time and B. Initiation of Narrow Basin-Centered Michigan extension (Sleep et al., 1980). This Late spatial pattern from the much earlier Keweenawan Subsidence Cambrian event perhaps represents a final Cam- rift. The Keweenawan rift (ca. 1100 Ma, Fig. 1) is brian extensional phase of the Reelfoot aulaco- well defined by gravity and seismic data and ex- Basin-centered subsidence began with deposi- gen, an interior counterpart to the divergent mar- tends northwest-southeast across the central basin tion of the Shakopee Formation (Lower to Middle gin that developed on the eastern and southern (Catacosinos and Daniels, 1991), effectively or- Ordovician). Although the Shakopee-Oneota con- margins (present coordinates) of the Laurentian thogonal to the northeast-southwest–trending tact is unconformable in Wisconsin (Smith et al., continent (Rankin, 1976). Later units within se- trough-shaped pattern of subsidence established 1993), there is no evidence for uplift and erosion of quence A, particularly the Trempealeau and for sequence A. The long wavelength and ampli- the underlying Oneota Formation within the cen- Oneota Formations, display slight and gradual tude of the large, regional gravity anomaly shown tral Michigan basin, as would be expected during thickening toward the basin center and overstep in Figure 1 suggest that it is located deeper in the lithospheric heating for thermal contraction and the underlying units toward the basin margins. crust than the narrow, high-amplitude gravity metamorphic phase change mechanisms proposed This pattern is compatible with thermal contrac- anomaly of the Keweenawan rift (Haxby et al., for the basin. tion following the moderate lithospheric exten- 1976). Sleep et al. (1980) suggested a depth of ori- Lithospheric stretching (McKenzie, 1978;

986 Geological Society of America Bulletin, July 1999 STRUCTURAL SEQUENCES AND STYLES OF SUBSIDENCE, MICHIGAN BASIN

Figure 12. Model for subsidence caused by stress-induced crustal weakening. Each panel shows the location of an excess mass in the upper crust and behavior of different lithospheric layers; on the right is a stylized lithospheric strength profile (after Ellis, 1988). (A) Under low stress condi- tions, the excess mass is flexurally supported by the entire lithospheric thickness. (B) High stress levels cause a weakening of the lithospheric por- tions where strength is controlled by crystal plasticity. Here the lower crust becomes sufficiently viscous to flow and the lithosphere-asthenosphere boundary is elevated. This allows the excess mass to be supported by the upper crust alone, resulting in narrow (low rigidity), basin-centered sub- sidence accompanied by lower crustal flow toward the basin margins, causing uplift of the surrounding arches. The total amount of flexure shown in this figure is exaggerated for clarity; total sediment accumulation during the two episodes of narrow, basin-centered subsidence in the Michi- gan basin is <2 km.

Klein and Hsui, 1987) is ruled out by the absence enough to allow viscous flow in the lower crust that the crust subsides due to an effective atten- of syndepositional faulting, whereas lithospheric (Fig. 12B). uation of the lower crust. Isostatic equilibrium flexure amplification due to in-plane stress Numerous studies of crustal rheology sup- is maintained throughout this process, so there (Cloetingh et al., 1985; Karner, 1986) and mod- port this model of lower crustal weakness is no rebound effect when the stress is removed. els of stress-induced basin reactivation (DeRito (Molnar, 1988) and quantitative models have Tectonic events capable of providing the hori- et al., 1983; Lambeck, 1983) also predict reacti- shown the impact of intraplate compressive zontal stresses to drive subsidence may be vation of the Illinois basin (there was no such stresses on viscoelastic stress relaxation in the found in a temporal correlation between early event at this time; Heidlauf et al., 1986). A stress- lower crust (Kusznir, 1982; Kusznir and Park, tectonic events in the Taconic and ini- induced crustal weakening mechanism intro- 1984). When the lower crust is thus weakened, tiation of sequence B subsidence (Howell and duced by Howell and van der Pluijm (1990) is the upper crust effectively becomes a thin plate, van der Pluijm, 1990). compatible with the geologic evidence. In this leading to flexural subsidence of the upper model, a load in the middle to upper crust is sup- crust beneath the anomalous load. Lower C. Dynamic Tilting of Eastern North America ported flexurally by a thick, lithospheric plate crustal material flows away from the central (Fig. 12A). Basin-centered subsidence is initiated basin subsidence toward the basin margins, re- Basin-centered subsidence abruptly ceased in when high intraplate stresses weaken the crust sulting in uplift of the arches. The net result is the latter part of Middle Ordovician time. There

Geological Society of America Bulletin, July 1999 987 HOWELL AND VAN DER PLUIJM is no clear evidence for a basin-centered subsi- for either a first or a second heating event in the conformity architecture of sequence B, and there dence component in any of the units composing basin (Sleep and Sloss, 1978). is no corresponding subsidence event in the Illi- sequence C. Rather, the Michigan basin along nois basin (Heidlauf et al., 1986). with most of eastern North America tilted east- D. Broad Basin-Centered Subsidence ward toward the developing Taconic orogenic F. Renewed Broad Basin-Centered welt. Because the wavelength of this tilting is so Basin-centered subsidence resumed in the Silu- Subsidence long (~1000 km) and includes preexisting rian (sequence D), but with a broad (saucer basins and arches inland as far as Wisconsin and shaped) distribution. In Howell and van der Pluijm The style of subsidence changed in later Mid- Iowa (Cook and Bally, 1975), flexural down- (1990), these styles of basin-centered subsidence dle Devonian time (Traverse Group, sequence F) warping from marginal, supracrustal loading were not distinguished and both sequences B and to broad basin-centered subsidence for ~4 m.y. (Quinlan and Beaumont, 1984; Beaumont et al., D were considered to have the same probable ori- Analogous to sequence D, narrow subsidence in 1988) is rejected as a potential mechanism, al- gin. However, quantification of the flexural behav- sequence E should generate a similar component though it certainly played a role in subsidence ior of these sequences (Howell, 1993), combined of broad subsidence in sequence F. The ~200 m of the more proximal portions of the foreland with recognition of two subsequent structural se- thickness of sequence F, however, contrasts with (Diecchio, 1993). quences of basin-centered subsidence, indicates the ~900 m of sedimentation due to thermal con- Gurnis (1992a) proposed that dynamic topogra- that these represent different styles of subsidence traction that is suggested for this model. There is phy associated with subduction beneath Laurentia with different origins. also a problem accounting for this subsidence pat- was responsible for uplift and subsequent tilting of Thermal contraction associated with the lower tern with such a model—why is there no lag time the Appalachian region during Late Ordovician crustal thinning suggested for narrow basin- in the onset of thermal contraction, such as that time. Howell (1993) and Coakley and Gurnis centered subsidence is a possible mechanism to proposed for sequence C? At this time we simply (1995) further explored this interpretation for the explain this pattern of broad basin-centered recognize the problem, but not its resolution. Ordovician history of the Michigan basin. In the subsidence. Thermal contraction subsidence due Gurnis model, uplift of a viscous (nonflexural) to uniform lithospheric attenuation directly fol- G. Final Laurentia-Gondwana Collision forebulge associated with subduction accounts for lows an instantaneous stretching event (McKenzie, the erosional post-Sauk unconformity in the east- 1978). In the model presented here, however, at- Late Paleozoic sedimentation in the Michigan ern Appalachian region. As dip of the westward- tenuation is limited to the lower crustal region; for basin and surrounding regions was strongly af- subducting slab dynamically shallowed, tilting ex- instantaneous attenuation in the lower crust only, fected by the orogenic activity associated with fi- tended farther into the midcontinent. This tilting mantle geotherms would be elevated an amount nal suturing of Laurentia and Gondwana. Clastic lasted until Late Ordovician collision of the approximately equal to the crustal thinning minus sediments dispersed cratonward across the fore- Taconic arc with the Appalachian margin (Stanley the associated sediment loading, and the geother- land, far from their erosional origins. In Michigan, and Ratcliffe, 1985), culminating in uplift and ero- mal gradient across the thinned lower crust would we find an unusual, irregular isopach pattern for sion across the foreland region. Polarity of the sub- be significantly increased. Simple calculations the Late Devonian units (Fig. 10A), suggestive of duction under Laurentia as required in the Gurnis suggest that a thermal anomaly at depth causes a an anomalous tilting to the west. However, this (1992a) model is opposite to that proposed by delay of tens of millions of years before peak heat pattern of sediment accumulation may not truly some workers (e.g., Jacobi, 1981; Hatcher, 1989; flow (and thermal contraction subsidence) occurs: reflect the shape of the basin due to the significant Bradley, 1989), but paleomagnetic and paleogeo- t = l2/κ = (3 · 104)2/(l · 10–6) = 30 m.y.; where t is uncertainty associated with the paleobathymetry graphic arguments support subduction under the the time delay, l is the length scale of interest of the Devonian black shale units. Laurentian margin (van der Pluijm et al., 1990, (depth to the zone of lower crustal attenuation, in Bond and Kominz (1991) surveyed the residual 1995). Gurnis (1992a) initially proposed a meters), and κ is the thermal diffusivity of crustal subsidence of this interval for basins in western, 200–500 km lateral extent for this tilting event rocks (M2s–1; Turcotte and Schubert, 1982). This southern and eastern North America and found a based on regional isopach maps of Cook and Bally delay in thermal contraction approximates the du- pattern of excess subsidence throughout the re- (1975). Documentation in this study and Coakley ration of sequence C, where no significant compo- gion, as compared to a benchmark section in Iowa and Gurnis (1995) indicates that tilting reached nent of basin-centered subsidence is recorded. that was assumed to be tectonically stable. They westward ~1000 km from the Ordovician margin suggested that this unusual pattern of regional of Laurentia. The stratigraphy within sequence C E. Renewed Narrow Basin-Centered subsidence developed as a result of North Amer- may provide strong constraints for future model- Subsidence ica moving into a zone of dynamic downwelling ing efforts. and compression as the continents coalesced into Sequence C also demonstrates that the basin- A resurgence of narrow basin-centered subsi- a supercontinent configuration. The significant centered subsidence in Early to Middle Ordovi- dence from latest Silurian to Middle Devonian westward thickening shown here for these units cian time (sequence B) effectively ceased for a pe- time (sequence E) suggests a renewal of elevated strongly suggests that the subsidence at the time riod of ~30 m.y., a result incompatible with intraplate stress and subsidence due to lower was neither a simple basin-centered nor an east- thermal contraction as a mechanism for sequence crustal weakening. A possible tectonic event trig- ward-tilting model. B. Thermal contraction of continental lithosphere gering increased lithospheric stress is the final ac- Devonian subsidence was succeeded by a fi- may last from 100 to 300 m.y., depending on cretion of the Avalonian microcontinent to Lau- nal episode of eastward, continental tilting in the lithospheric thickness (Ahern and Ditmars, 1985). rentia, which correlates with a change in the Mississippian, reflecting final collision between Cessation of thermal contraction can only arise direction of Laurentian plate motion from south- Laurentia and Gondwana and the formation of from a second heating event, timed to add just ward to northward (van der Pluijm et al., 1990). Pangea (upper sequence G; Fig. 10, B and C). enough heat to the lithosphere to replace that be- Sequence E subsidence was accompanied by re- Although Alleghanian supracrustal loading ex- ing lost by conduction at the surface. This situa- gional uplift and erosion on the arches flanking tended significantly farther westward than ear- tion is unlikely by the lack of geologic evidence the basin in a fashion analogous to the un- lier Taconian thrusting, it is not clear that Mis-

988 Geological Society of America Bulletin, July 1999 STRUCTURAL SEQUENCES AND STYLES OF SUBSIDENCE, MICHIGAN BASIN sissippian tilting in Michigan is a flexural re- and Watts, 1983). Thus eastern Pennsylvania, like use of structural sequences provides a quality sponse to this load. Erosion of the basin margins, the Michigan basin, may have undergone subsid- check on basin subsidence data sets that will aid however, prohibits full examination of this prob- ence rather than exposure at the end of the Sauk the basin modeling process. Interpretation prob- lem because of the limited stratigraphic data sequence due to stress-induced crustal weaken- lems of several previous studies of the Michigan preserved. ing. In the broader context of the post-Sauk un- basin are largely resolved when the stratigraphy is conformity, most of eastern Laurentia had accu- examined in light of structural sequences, rather DISCUSSION mulated thick deposits of Cambrian and Lower than using a predetermined stratigraphic grouping Ordovician sediments at the time of this event. We (Fisher et al., 1988) or if paleobathymetry had This study of Michigan basin stratigraphy pre- suggest that these thick sediments would produce been explicitly considered (Coakley et al., 1994). sents further support for the geodynamic model a negative load (net buoyancy) in the upper crust Quinlan and Beaumont (1984) and Beaumont presented by Howell and van der Pluijm (1990) over a broad region when the lower crust became et al. (1988) produced a synthetic model of for basin-centered subsidence. A significant ques- weakened, and this would lead to modest uplift supracrustal loading on a viscoelastic plate that tion concerning this model is how to test it further. and erosion in synchrony with the subsidence of successfully reproduced the first-order stratigra- We suspect that seismic methods in search of a sequence B in the Michigan basin. Thus, erosion phy of eastern North America. However, the mod- thinned and deformed lower crust would be in- of the post-Sauk unconformity over the broad in- eling of Beaumont et al. (1988) achieved signifi- conclusive; the total amount of deformation is terior of eastern Laurentia might be related more cant regional subsidence as far inland as eastern small relative to the thickness of the crust (<1 km to in-plane stresses associated with the initiation Wisconsin during Middle Ordovician time only vs. ~40 km; Zhu and Brown, 1986) and imaging of the Taconic orogeny than to a migrating peri- through specifying an additional component of the lower crust in any detail with reflection seis- pheral bulge (Jacobi, 1981; Mussman and Read, basin-centered tectonic subsidence in Michigan mic tools is still a difficult task. 1986). The extent of the post-Sauk unconformity (the subsidence of the Michigan and Illinois basins More productive at this time will be efforts to goes well inboard on the , beyond the reach was not explained geodynamically, but rather was examine conceptual predictions about surface de- of a migrating peripheral bulge with any realistic simply added to the model based on empirical sed- formation made by this subsidence model, and to lithospheric rigidities, but in-plane stresses may iment thickness data). In large part, the authors compare them with the stratigraphic record. The transfer ~2000 km into a plate interior (Craddock specified this additional subsidence in their model first prediction is that arches surrounding the et al., 1993; van der Pluijm et al., 1997). Similarly, in order to fit the isopach pattern of the entire lower basin should be uplifted during narrow, basin- the narrow, basin-centered subsidence of se- Tippecanoe sequence of Sloss (1963), which in- centered subsidence due to outward flow of quence E corresponds in time to erosion of the cludes the St. Peter Sandstone (sequence B, basin lower crustal material. This effect is seen along widespread post-Tippecanoe sequence boundary centered) with the Black River and Trenton the Kankakee and Algonquin arches during Or- of Sloss (1963). In-plane stress models may pro- Formations (sequence C, tilting). The subsidence dovician time (sequence B subsidence) and again vide a new framework with which to examine the due to supracrustal loading alone (without the Illi- in this area during Late Silurian to Early Devo- origin of the widespread Sloss sequences and nois or Michigan basins added) would have been nian time (sequence E subsidence). Second, the bounding unconformities (cf. dynamic topogra- insufficient to explain the observed stratigraphic shape of the basin during narrow, basin-centered phy model of Burgess et al, 1997). thickness of the Black River–Trenton succession subsidence should reflect flexure of a thin, crustal We reiterate here and extend our previous sug- in the cratonward portion of their study area. High- plate instead of a thick, lithospheric plate. Pre- gestion (Howell and van der Pluijm, 1990) that the resolution modeling points out this geodynamic liminary results of flexural modeling suggest ef- initiation of Taconic orogenesis correlates tempo- dilemma, which has subsequently been used as ev- fective plate thicknesses for sequences B and E of rally with initiation of sequence B. High levels of idence for subduction-related dynamic topography ~30 km, compared with ~70 km for sequence D intraplate stress seem to be required to produce ef- (Gurnis, 1992a; Howell, 1993; Coakley and and ~80 km for sequence F episodes of broad, fects such as we describe here, levels that must be Gurnis, 1995). basin-centered subsidence (Howell, 1993). These only rarely achieved by tectonic processes. values suggest flexure of the entire lithosphere Stresses associated with simple subduction of CONCLUSIONS during broad, basin-centered subsidence, and oceanic lithosphere beneath a continent seem in- flexural decoupling with only the upper crust sufficient to generate the widespread effects we Analysis of structural sequences significantly (~30 km) supporting the surficial loading during document here (cf. Cloetingh and Wortel, 1986, aids interpretation of basin geometry through narrow, basin-centered subsidence. Coblentz et al., 1995). During the main phases of time, and has applicability in other cratonic areas. A third prediction of this model concerns the Middle to Late Ordovician forma- The Michigan basin underwent several distinc- nature of surficial deformation that might occur tion and Taconic deformation in the northeastern tive changes in basin subsidence patterns, reflect- across eastern Laurentia during an episode of United States, Laurentia is tilting eastward with no ing changes in the controlling subsidence mech- stress-induced crustal weakening. If the lower basin-centered subsidence in Michigan. Rather, anisms. These changes in basin subsidence, crust becomes weakened over this broad region, we suggest that episodes of significant collision or combined with the architecture of unconformities then other areas with significant upper crustal plate reorganization (such as would likely occur at associated with the structural sequences, together loads may subside, and areas with extra light the very beginnings of significant ) lead provide important constraints on viable subsi- loads in the upper crust may undergo uplift and to tectonic conditions that impart high levels of in- dence mechanisms for the Michigan basin. Al- erosion. For example, eastern Pennsylvania has a plane stress to cratonic lithosphere. though not a quantitative geodynamic model, the significantly thickened lower Middle Ordovician We have demonstrated the utility of a structural stress-induced subsidence mechanism of Howell carbonate succession and is apparently lacking a sequence approach to examining basin subsidence and van der Pluijm (1990) is further supported by post-Sauk unconformity (Mussman and Read, history. By utilizing structural sequence criteria this expanded database for two episodes of nar- 1986), and there is a significant Bouguer gravity that carefully define basin deformation patterns row, basin-centered subsidence. A stress-induced anomaly in that area that does not match the typi- and further demand consideration of the effects of crustal weakening mechanism for subsidence of- cal foreland basin paired anomaly pattern (Karner paleobathymetry, compaction, and dissolution, the fers several opportunities for further testing and

Geological Society of America Bulletin, July 1999 989 HOWELL AND VAN DER PLUIJM

Calvert, W. L., 1974, Sub-Trenton structure of , with views Harrison, W. B., III, 1985, Lithofacies and depositional envi- evaluation based on stratigraphic data. These on isopach maps and stratigraphic sections as basis for ronments of the Burnt Bluff Group in the Michigan basin, findings, in combination with other models link- structural myths in Ohio, Illinois, New York, Pennsylva- in Cercone, K. R., and Budai, J. M., eds., Ordovician and ing plate dynamics with stratigraphy of plate in- nia, West Virginia, and Michigan: American Association Silurian rocks of the Michigan basin and its margins: of Petroleum Geologists Bulletin, v. 58, p. 957–972. Michigan Basin Geological Society Special Paper 4, teriors, suggest that we can look at cratonic re- Catacosinos, P. A., and Daniels, P. A., 1991, Stratigraphy of p. 95–108. gions for clues to understanding the complex Middle to Middle Ordovician formations of Harrison, W. B., III, 1987, Michigan’s “deep” St. Peter gas play tectonic history of ancient plate interactions. the Michigan basin, in Catacosinos, P. A., and Daniels, continues to expand: World Oil, April, p. 56–61. P. A., eds., Early sedimentary evolution of the Michigan Hatcher, R. D., Jr., 1989, Tectonic synthesis of the U.S. Ap- basin: Geological Society of America Special Paper 256, palachians, in Hatcher, R. D., Jr., Thomas, W. A., and ACKNOWLEDGMENTS p. 53–71. Viele, G. W., eds., The Appalachian-Ouachita orogen in Catacosinos, P. A., Daniels, P. A., and Harrison, W. B., III, the United States: Boulder, , Geological Society 1991, Structure stratigraphy, and petroleum of of America, Geology of North America, v. F2, p. 511–535. This paper forms part of Howell’s dissertation the Michigan basin, in Leighton, M. W., Kolata, R. D., Haxby, W. F., Turcotte, D. L., and Bird, J. M., 1976, Thermal at the University of Michigan. We thank Joyce Oltz, D. F., and Eidel, J. J., eds., Interior cratonic basins: and mechanical evolution of the Michigan basin: American Association of Petroleum Geologists Memoir Tectonophysics, v. 36, p. 57–75. Budai, Mike Gurnis, Art Leibold, Henry Pollack, 51, p. 561–601. Heidlauf, D. T., Klein, G. deV., and Hsui, A. T., 1986, Tectonic Brian Whiting, Bruce Wilkinson, and Mark Kulp Célérier, B., 1988, Paleobathymetry and geodynamic models subsidence analysis of the Illinois basin: Journal of Geol- for discussion and comments on earlier versions, for subsidence: Palaios, v. 3, p. 454–463. ogy, v. 94, p. 779–794. Cloetingh, S., and Wortel, R., 1986, Stress in the Indo-Aus- Hinze, W. J., Kellogg, R. L., and O’Hara, N. W., 1975, Geo- and Bill Harrison, Michelle Kominz, and Jeff tralian plate: Tectonophysics, v. 132, p. 49–67. physical studies of basement geology of southern penin- Nunn for helpful reviews of the manuscript; how- Cloetingh, S., McQueen, H., and Lambeck, K., 1985, On a tec- sula of Michigan: American Association of Petroleum tonic mechanism for regional sea level variations: Earth Geologists Bulletin, v. 59, p. 1562–1584. ever, they may not all necessarily agree with the and Planetary Science Letters, v. 75, p. 157–166. Houseman, G. A., and England, P. C., 1986, A dynamical ideas presented in this paper. We acknowledge fi- Coakley, B., and Gurnis, M., 1995, Far-field tilting of Lauren- model of lithosphere extension and nancial support from the Rackham Graduate tia during the Ordovician and constraints on the evolution formation: Journal of Geophysical Research, v. 91, of a slab under an ancient continent: Journal of Geophys- p. 719–729. 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