Debra L. Hanneman

Sequence stratigraphy of Cenozoic continental rocks, southwestern Montana

Because the presently used Bozeman Group lithostratigraphy is difficult to apply to Cenozoic basin-fill rocks in the Jefferson, Beaverhead, Melrose, and Divide Valleys, southwestern Montana, we advocate the use of sequence stratigraphy. Surface sequence boundaries are in many cases marked by calcic paleosol zones; these zones can be projected into the subsurface and tied to seismic reflection data. Density/velocity contrasts between calcic paleosol zones and overlying nonpedogenic rocks generate bright reflectors that define sequence boundaries. Sequence boundary delineation is further enhanced by reflection termination patterns, similar to those found in marine strata. Sequence age calibration is based upon vertebrate fossil and radiometric age data. Five sequences, separated by basin-wide unconformities, are recognized. Most sequence-bounding unconformities are marked by paleosols, an occurrence not previously reported for Cenozoic strata of southwestern Montana. The sequences and their age ranges are (1) Bridgerian-Uintan (approximately 50 to 44 Ma) calc-alkaline volcanic flows and interstratified sedimentary rocks, (2) Duchesnian to Whitneyan (approximately 42 to 30 Ma) terrestrial sedimentary and volcanic rocks, (3) Arikareean (approximately 27 to 21 Ma) terrestrial sedimentary and volcanic rocks, (4) Barstovian to Blancan (approximately 16 to 4 Ma) terrestrial sedimentary and volcanic rocks, and (5) early Quaternary (approximately 1.8 Ma) to Holocene terrestrial sedimentary and volcanic rocks.

Calcic Paleosols: Their Use in Subsurface Stratigraphy

Analysis of well log data from Cenozoic basin fill of the Deer Lodge Valley, southwestern Montana, provides evidence for identifying paleosols and paleosol stacks in the subsurface. The paleosol stacks are continental sequence boundary markers and appear as several relatively thin, high-velocity/high-density zones within basin fill. Zone thickness ranges from 1 to 1.5 m; zones are stacked to thicknesses of up to 15 m. Density varies within the zones by as much as 0.6 g/cm3, and differs by as much as 0.9 g/cm3 from material immediately above these zones. Velocity differs by as much as 10 ft/ms from the overlying material and causes bright reflections on seismic sections. Synthetic seismograms are used to tie well log and seismic data. Basing our interpretation upon well log data and well cuttings analyses, we determined the high-velocity/high-density zones to be limestone. The pedogenic origin of the limestone is shown by (1) well cutting chips from the high-velocity/high-density zones that exhibit pedogenic features associated with calcic paleosols, (2) paleosol horizonation interpreted from well log analysis, (3) the absence of minerals normally associated with lacustrine deposits, and (4) comparison with surface paleosol exposures.

Paleosols: Reflectors in continental sequences

The study of ancient soils (paleosols) is not usually a discipline associated with seismic exploration. However, we have found that paleosols are excellent stratigraphic markers in surface exposures and form distinctive reflection packages on seismic data from continental Cenozoic areas.

Chapter 5 Continental Sequence Stratigraphy and Continental Carbonates

Abstract Although originally developed in the marine realm, sequence stratigraphy can be applied to continental settings that include carbonates from soil forming, palustrine, lacustrine, and groundwater environments. Continental sequence stratigraphy provides a basis for correlation of frequent lateral and vertical facies changes, a framework for interpretation of continental deposits, and predictive models for facies geometry and reservoir architecture. The subaerial unconformity, the maximum flooding surface, and the correlative conformity are the three basic surfaces suitable for bounding units in continental sequence stratigraphy. Other sequence stratigraphic terminology commonly used in marine sequence stratigraphic work such as systems tracts and parasequences can be applied to continental sequences, but it may be more applicable to certain depositional environments that mimic some marine processes, such as lacustrine settings. Continental carbonates are extremely useful in sequence stratigraphic studies as shown by numerous case studies reviewed in this chapter. The most commonly used carbonates for sequence boundary delineation are mature calcic paleosols/calcic paleosol pedocomplexes and palustrine deposits. In some settings, palustrine carbonates are so overprinted by pedogenesis that both types of carbonates essentially mark the same sequence boundary. Nevertheless, both calcic paleosols/pedocomplexes and palustrine calcretes are typically associated with regional surfaces and possess physical properties that make them easily identifiable on the surface and in the subsurface. Controls on the formation of continental carbonates within a sequence stratigraphic framework are varied and often are interrelated. Although carbonate-bearing sequences and carbonates that delineate sequence boundaries are mainly controlled by tectonism and climate, eustasy may play a major role in coastal plain settings. Many case studies are presented in this chapter that document the role of tectonism, climate, and eustasy in the sequence stratigraphy of continental carbonates.

CENOZOIC STRATIGRAPHY IN WESTERN MONTANA: CURRENT STATUS AND A WAY FORWARD

Lithostratigraphy is an approach currently used by some for Cenozoic stratigraphy in western Montana. Cenozoic lithostratigraphic units are part of the Bozeman Group, with lower Tertiary strata placed in the Renova Formation and upper Tertiary deposits comprising the Sixmile Formation. The characterizing lithologic features of this two-fold formal lithostratigraphy are that strata within the Renova Formation are predominantly fine-grained and the Sixmile Creek Formation contains mainly coarsegrained rocks. The difficulty in applying this stratigraphy is that basin strata lithology commonly exhibit abrupt lateral lithologic changes, with predominantly fineand coarse-grained units being often repeated throughout the stratigraphic column. Consequently, Bozeman Group lithostratigraphy does not provide a solid mappable basis for working with Cenozoic strata and has resulted in muddled geologic interpretations for these basins. A way forward for western Montana Cenozoic stratigraphy is the use of sequence stratigraphy. Cenozoic continental strata in western Montana basins can be separated into five sequences that can be delineated in the field based upon unconformable relationships with other strata and by their capping of mature paleosols. In ascending order, approximate age ranges for these informally designated sequences are: (1) ~54 to 44 Ma, (2) ~38 to 32 Ma, (3) ~27 to 21 Ma, (4) ~16 to 4 Ma, and (5) ~2 Ma to present time. A regional unconformity separates an early Eocene paleolandscape from the overlying five Cenozoic sequences. The early Eocene unconformity and Sequence 1 are delineated by ultisols; all other younger sequence boundaries have bounding calcic paleosols. The sequences are presently age constrained by existing isotopic ages and by North American Land Mammal ages derived from contained fossil vertebrate assemblages. A new effort to more tightly age constrain the sequences includes six 40Ar/39Ar isotopic ages ranging from about ~ 28 Ma to 2 Ma for tuffs in the Blacktail-Ruby valleys, southwest Montana, and ten tuff samples from other southwest Montana localities that are currently undergoing isotopic age analysis. Ultimately, Cenozoic sequence stratigraphy provides a better framework for both interand intra-valley correlation and yields a more accurate basin formation time frame. Paleosol profiles at sequence boundaries are often stacked and comprise pedocomplexes where two or more paleosols are separated over large areas by a thin deposit of C horizon material, and where they are overlain and underlain by larger amounts of strata that contain weak to no evidence of soil development. Calcic Pedocomplex Calcic paleosols mark the Sequences 2/4 boundary in the lower Madison Valley, southwestern Montana. Sequence 2 mudstone and cryptic grus channels, Little Pipestone area, Jefferson Valley. Sequence 1 ash-flow tuff, Deer Lodge Valley. Sequence 1 conglomerate and red mudstone, Sage Creek Valley. Sequence 3 paper shale overlain by calcic paleosol,Townsend Valley. Sequence 4 mudstone near Whitehall, in the Jefferson Valley. Sequence 3 conglomerate and coarse-grained sandstone strata, Toston Valley (Sixmile Creek type section). Blacktail Ruby 40Ar/39Ar Isotopic Ages and Locality Data @36-25 Ma Tertiary Locality @16-10 Ma Tertiary Locality Insect Beds