Importance of high-frequency tectonic sequences during greenhouse times of Earth history

Boyan K. Vakarelov Geosciences Department F021, University of Texas at Dallas, P.O. Box 803688, Richardson, Texas 75083-0688, USA Janok P. Bhattacharya Geosciences Department, SR1 Rm 312, University of Houston, 4800 Calhoun Rd., Houston, Texas 77204-5007, USA Dragana D. Nebrigic Geosciences Department F021, University of Texas at Dallas, P.O. Box 803688, Richardson, Texas 75083-0688, USA

ABSTRACT trates the first geochronologically and biostra- The relative importance of tectonism in the stratigraphic record should be more clearly tigraphically constrained example, which expressed during greenhouse times of Earth history due to the lack of overmasking effects shows high-frequency tectonic control in a of high-frequency and high-amplitude sea-level changes typical for icehouse periods. Es- stratigraphic interval during a greenhouse tablishment of the importance of tectonism, especially at temporal scales comparable to time. We build on a detailed allostratigraphic durations of typical transgressive-regressive cycles, has been plagued by poor temporal framework developed by Bhattacharya and resolution. Our outcrop and subsurface-based study of a Cenomanian shallow-marine Willis (2001), as well as comparisons with late siliciclastic interval, constrained by bentonite-based geochronology and detailed biostra- Cenozoic and Quaternary high-frequency tec- tigraphy, examines this problem. In a 2.2 m.y. interval, we identified four tectonically tonic analogs (e.g., Fortuin and DeSmet, 1991; driven erosional surfaces that dominate preserved stratigraphy. Biostratigraphic corre- Ito et al., 1999). lation to a sea-level curve for the Cenomanian—where coeval high-frequency low- amplitude eustatic cyclicity has been demonstrated—allows the first direct comparison of STUDY AREA the effects of eustasy and tectonics at temporal scales of hundreds of thousands of years The study area, located on the western flank during a global greenhouse time. We suggest that minor tectonic pulses locally overshadow of the Powder River Basin (east-central Wy- the effects of eustasy and exert the dominant control over preserved stratigraphy. While oming) (Fig. 1), was situated in a shallow subtle tectonic control on sedimentation has been documented throughout the Cretaceous overfilled distal foreland to cratonic setting Western Interior, the results of this study suggest that much of that deformation occurred during Cenomanian time. The area was sub- at sub-million-year frequencies and at time scales comparable to eustatic transgressive- sequently affected by large-scale basement- regressive cycles. involved deformation during the Laramide orogeny, which reached eastern Wyoming by Keywords: tectonic controls, Milankovitch theory, sequence stratigraphy, biostratigraphy, ero- the early Tertiary (Crowley et al., 2002). The sion surfaces, foreland basins. Frontier Formation has been subdivided into three unconformity-bounded members—Belle INTRODUCTION gested that long Milankovitch-frequency eu- Fourche, Emigrant Gap (locally missing), and There remains a significant debate as to static variation (400 k.y.) was a factor during Wall Creek Member, which span ϳ8 m.y. whether sequence-bounding unconformities the Cenomanian (Gale et al., 2002). Because (Merewether, 1996). These unconformities, are dominated by tectonic, eustatic, or other both eustasy and tectonics can independently identified by absence of molluscan fossil controlling mechanisms, such as climatic var- generate high-frequency sequences, external zones, have been interpreted to be of tectonic iation (e.g., Van Wagoner, 1995; Yoshida et criteria, such as recognition of diagnostic origin and recur with frequencies of several al., 1996). While Quaternary sediments are stratigraphic architecture or correlation of co- million years (Merewether and Cobban, better suited for resolving such issues due to eval base-level changes at far-removed local- 1986). Regional outcrop and well-log–based typically excellent time control (e.g., Ito et al., ities, must be used to distinguish tectonic from mapping within the lower portion of the Belle 1999), such systems are poorly representative eustatic stratigraphic cycles. Our study illus- Fourche Member has defined four distinct of greenhouse times in Earth history, when amplitudes of sea-level variation were much lower (Gale et al., 2002). Resolving the im- Figure 1. Map showing positions of Frontier For- portance of tectonic and eustatic control di- mation outcrop belt (light rectly from ancient data is problematic, since gray), well-log cross sec- –depositional events often occurred at time tions (30–30؅, 40–40؅, 50 scales shorter than available temporal resolu- 50؅), location of Second Frontier truncation in out- tion (Miall, 1992). The Cretaceous section of crop (black star), regional the Western Interior of North America was Second Frontier erosion- one of the first places where sub-million-year al trend identified in sub- coarsening-up cycles (parasequences) were in- surface (dashed gray line), and Tisdale Anti- vestigated, resulting in various hypotheses for cline. Triangles, stars, their generation. Some workers suggested and squares displayed on domination of glacio-eustasy (e.g., Plint, cross-sections 30–30؅ Van Wagoner, 1995), while others pre- and 40–40؅ correspond to ;1991 well logs shown in Fig- ferred tectonic forcing (Yoshida et al., 1996). ures 3A, 3B, and 3C. More recent high-resolution work has sug-

᭧ 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; September 2006; v. 34; no. 9; p. 797–800; doi: 10.1130/G22348.1; 6 figures. 797 Figure 2. Examples of tectonically driven erosion of Second Frontier Sandstone in outcrop and subsurface. Top: Eight measured sections along continuous outcrop show increased removal of high-energy facies to left. Bottom right: Locations of measured sections on aerial photograph in same order from left to right. Bottom left: Expression of same erosional event in resistivity well logs from portion of cross- .section 50–50؅ discontinuity-bounded allomembers: Harlan, sional surfaces within a high-resolution chron- both outcrop and closely spaced well-log data. Willow, Frewens, and Posey, from lowest to ostratigraphic framework. The Harlan allo- The regional trends of these truncations ap- highest (Bhattacharya and Willis, 2001). Each member overlies the Clay Spur Bentonite pear linear and parallel to modern structural allomember is composed of an upward- dated at 97.17 Ϯ 0.69 Ma (40Ar/39Ar, sani- features, which further supports a tectonic coarsening mudstone-to-sandstone lithofacies dine; Obradovich, 1993) and underlies Ben- interpretation. Additionally, no significant succession erosionally capped by a chert- tonite 5, dated at 95.86 Ϯ 0.45 Ma (40Ar/39Ar, coarse-clastic accumulations have been pebble lag, which has been interpreted as a sanidine; Obradovich, 1993). The Willow al- mapped ‘‘seaward’’ of the trend of these ero- detached lowstand deltaic deposit (Bhatta- lomember, the Posey allomember, and the sional surfaces, which might be expected if charya and Willis, 2001). These units are overlying Second Frontier Sandstone are these were river-cut valleys. overlain by the coarsening-up succession of capped by the Soap Creek Bentonite, dated at An outcrop example is shown in the Second the Second Frontier Sandstone, which is also 94.93 Ϯ 0.53 Ma (40Ar/39Ar, sanidine; Obra- Frontier Sandstone (Fig. 2, see star in Fig. 1). capped by a pebble lag. dovich, 1993). Bentonite ages and biostrati- Eight measured sections along an ϳ1.3-km- Seven regionally extensive bentonite beds graphic control allow placement of strati- long exposure show the progressive south- (three dated) allow placing of mapped ero- graphic units and erosional surfaces within a ward removal of high-energy, coarse-grained high-resolution temporal framework. shoreface facies. Truncation occurs under- neath a flat erosion surface that overlies EVIDENCE FOR TECTONIC ORIGIN medium-grained, large-scale cross-bedded OF FOUR BELLE FOURCHE shore face sandstones in the north, and finer- grained, planar to hummocky cross-stratified, UNCONFORMITIES mud lens–bearing lower shoreface sandstones Several criteria suggest that four of the ero- in the south. The erosional surface is identified sional surfaces mapped in the study are tec- by a pebble lag, which is overlain by trans- tonically driven. Rather than representing a gressive marine pebble-bearing muddy depos- marine-to-nonmarine contact, as would be ex- its. The facies directly underlying the erosion- pected if erosion was due to valley incision, al surface in the southern portion of the these surfaces show characteristics typical of outcrop appear similar to facies ϳ10 m below a transgressive surface eroded by waves, such the erosional surface in the north (Fig. 2). We as presence of the Glossifungites ichnofacies interpret such variation in depth of erosion as (firm-ground burrowing organisms commonly related to a higher degree of wave erosion associated with transgressive erosion surfaces; over a paleohigh area to the south. An addi- MacEachern et al., 1992), shark-teeth–bearing tional outcrop-based example, displaying a pebble lags, and overlying offshore marine de- similar style of differential erosion over a pa- posits (Cattaneo and Steel, 2003). Such sur- leohigh area, was mapped in the Harlan allo- faces do not show undulating channelized ge- member of the lower Belle Fourche (see Fig- Figure 3. Examples of tectonic truncations ometries, are never overlain by nonmarine ure 4b in Bhattacharya and Willis, 2001), in Harlan (C), Willow (B), and Posey (A) al- fluvial channel or floodplain facies, and can where up to 25 m of sediments was interpreted lomembers from portions of resistivity well- overlie upper shoreface, lower shoreface, or to have been removed. In both examples, ero- -log cross-sections 30–30؅ and 40–40؅. Lo cations of well logs are shown in Figure 1 offshore facies. The lateral variation of ero- sional surfaces appear marine in origin, dis- (see figure caption for details). H—Harlan, sion depth results in abrupt truncation of sand- play flat geometries, and suggest wave re- W—Willow, P—Posey. dominated intervals, which are mappable in working as an erosional mechanism.

798 GEOLOGY, September 2006 erosional surfaces show abrupt thinning and The four tectonically driven erosional sur- removal of sand-rich successions, truncation faces occur in a succession that was previous- of well-log markers, and a NNW-SSE ero- ly considered to be conformable and that un- sional trend, similar to the Second Frontier derlies a prominent regionally mappable 2 and Harlan examples (Fig. 4). The mapped m.y. unconformity (see Figure 10 in Mere- erosion surfaces suggest four individual pulses wether and Cobban, 1986) (Fig. 6), which has of tectonic movement over a time span of also been interpreted to be of tectonic origin ϳ2.2 m.y. (Merewether and Cobban, 1986). The first tec- tonically driven unconformity truncates por- DISCUSSION tions of the Harlan allomember, and occurs in The chronometric control from the benton- the upper portion of the ammonite-free inter- ites (Obradovich, 1993) and the biostrati- val, positioned between the Clay Spur Ben- graphic control allow us to compare the rela- tonite and Bentonite 5, a period of 1.31 Ϯ 0.8 Figure 4. Fence-diagram showing preserved tive importance of tectonics and eustasy at sand-dominated portions of allomembers sub-million-year frequencies. Detailed bio- m.y. (Figs. 5 and 6). The other three uncon- (dotted pattern) and extent of tectonically stratigraphy allows correlation to a high- formities, topping the Willow, Posey, and Sec- driven erosion (dashed lines) in well-log resolution global sea-level curve for the Cen- ond Frontier units, respectively, occur in the .cross-sections 50–50؅, 40–40؅, and 30–30؅ omanian (Gale et al., 2002) (see Figs. 5 and ammonite-bearing Bentonite 5–Soap Creek H—Harlan, W—Willow, P—Posey, SF— Bentonite interval over a period of 0.93 Ϯ 0.7 Second Frontier Sandstone. See Figure 1 for 6). An exceptionally high level of biostrati- locations. graphic correlation was achieved by the use of m.y. (Figs. 5 and 6). The number of erosional widely distributed ammonite species belong- surfaces per bentonite-bounded interval, taken ing to rapidly evolving Acanthoceratacae and in conjunction with biostratigraphic events Closely spaced well logs penetrating the Turrilitacea superfamilies. Relatively com- within ammonite fauna (e.g., the first and last Second Frontier Sandstone (Fig. 2) and the plete ammonite-bearing Cenomanian succes- occurrence) suggest a sub-million-year rate of Harlan allomember (Fig. 3C) allow these ero- sions of the study area yield 13 ammonite bio- recurring of tectonic movements. sional surfaces to be traced in the subsurface zones (Merewether and Gautier, 2000), which Numerical modeling results suggest that in- and to be mapped regionally. Differential ero- are plotted against co-occurring ammonite traplate stresses in plate interiors are sufficient sion is indicated by an abrupt lateral decrease biozones of the Anglo-Paris Basin (Gale et al., to generate significant vertical movements at in thickness of sand-dominated successions 2002). The ranges of all species are not iden- the scale and frequency observed in this study and by truncation of well-log markers, includ- tical in both basins, but their relative orders (Cloetingh, 1988; Peper et al., 1992; Heller et ing the bentonites. Erosional trends are linear are the same, and the pattern of occurrence is al., 1993). While examples of subtle tectonic and subparallel to modern Laramide structures analogous. Since the lower part of the Belle control on sedimentation from the Western In- (see Fig. 1), suggesting basement-fault con- Fourche Member is missing important bio- terior of the United States are numerous (e.g., trol. We identified and mapped two additional stratigraphic markers, only the interval that Heller et al., 1993; Zaleha et al., 2001), little erosion surfaces at the tops of the Willow and spans the middle to upper Cenomanian (cor- information is available on recurrence rates. Posey allomembers in the subsurface to the responding to sequences 6–12 of Anglo-Paris Extracting a tectonic signal from a bentonite- east of the outcrop belt (Figs. 3A–3B). All Basin) was used (Fig. 6). bearing fossiliferous shallow-marine interval

Figure 5. Resistivity well-log cross-section 50–50؅ showing distribution of seven regionally recognized bentonites (thick gray lines), sandy portions of allomembers (dotted pattern), and tectonically driven erosional surfaces (dashed lines). Well locations on Figure 1 are deter- mined by using number overlying each well and counting toward NE. Ages for the Clay Spur Bentonite (CS), Bentonite 5, and Soap Creek Bentonite (SC) are from Obradovich (1993).

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Manuscript received 9 February 2006 Heller, P.L., Beekman, F., Angevine, C.L., and Cloe- Revised manuscript received 6 April 2006 ACKNOWLEDGMENTS tingh, S., 1993, Cause of tectonic reactivation and Manuscript accepted 11 April 2006 The study has been made possible by consortium subtle uplifts in the Rocky Mountain region and support from Chevron, British Petroleum, and Anadar- its effect on the stratigraphic record: Geology, Printed in USA

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