Evidence of a lacustrine origin for laminated marl of the Pliocene Bouse Formation, Milpitas Wash, Blythe Basin, lower Colorado River valley J.E. Spencer, K. Constenius, J. Bright

December 2018 CONTRIBUTED REPORT CR-18-K Arizona Geological Survey azgs.arizona.edu | repository.azgs.az.gov Arizona Geological Survey

Phil A. Pearthree, Director

Printed by the Arizona Geological Survey All rights reserved

Arizona Geological Survey visit azgs.arizona.edu

- sity of Arizona, or any department thereof, or any of their employees, makes no warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, purposes only and does not imply endorsement by the University of Arizona

Arizona Geological Survey Contributed Report series provides non-AZGS authors with a forum for publishing documents concerning Arizona geology. While review comments may have been incorpo- rated, this document does not necessarily conform to AZGS technical, editorial, or policy standards. The Arizona Geological Survey issues no warranty, expressed or implied, regarding the suitability respect to claims by users of this product. The author(s) is solely responsible for the data and ideas expressed herein. ______

Colorado River valley. Arizona Geological Survey Contributed Report, CR-18-K, v 1.1, 33 p. Evidence of a lacustrine origin for laminated marl of the Pliocene Bouse Formation, Milpitas Wash, Blythe Basin, lower Colorado River valley

2018

Arizona Geological Survey Contributed Report CR-18-K

version 1.1

Jon E. Spencer, Department of Geosciences, The University of Arizona, 1040 E. 4th St., Tucson, AZ 85721, USA ([email protected])

Kurt Constenius, Department of Geosciences, The University of Arizona, 1040 E. 4th St., Tucson, AZ 85721, USA ([email protected])

Jordon Bright, Department of Geosciences, The University of Arizona, 1040 E. 4th St., Tucson, AZ 85721, USA ([email protected])

Abstract

The Pliocene Bouse Formation in the lower Colorado River trough locally contains laminated marl and claystone interpreted by O’Connell et al. (2017) to represent the spring-neap tide cycle during sediment deposition in an estuary. Tidal cycles, if present, should be detectable by Fourier spectral analysis of lamination thicknesses in continuous sequences of laminated sediments. To evaluate the tidal interpretation, we attempted to photograph several laminated sequences in the southern Bouse Formation (south of Blythe, California) so that thicknesses could be measured from the photographs. Only one sequence, in lower Milpitas Wash (California), was identified where thicknesses could be determined with adequate precision from field photography. Fourier analysis of that sequence failed to identify evidence of tides. Furthermore, electron-microprobe analysis determined that laminations consist of alternating claystone and marl, which is consistent with annual changes in lake chemistry and sediment sources rather than physical changes in sediment sorting and transport during tidal cycles. Fourier analysis of data presented by O’Connell et al. (2017) of two nearby laminated Bouse sequences interpreted as tidal rhythmites also failed to identify statistically significant evidence of tides.

1

Bouse Formation

The lower Pliocene Bouse Formation in the lower Colorado River valley of southeastern California, western Arizona, and southernmost Nevada consists typically of 1–10 m of basal carbonate (travertine/tufa, limestone, marl, calcareous sandstone) overlain conformably by mudstone, siltstone and fine sandstone. These strata are in turn overlain unconformably by Colorado River sand and gravel or alluvial fan sediments (Metzger et al., 1973; Metzger and Loeltz, 1973; Buising, 1990; House et al., 2008; Pearthree and House, 2014; Homan, 2014). The Bouse Formation onlaps onto alluvial fan sediments, bedrock hillslopes, and in a few exposures, finer grained axial valley deposits, and represents abrupt inundation of a previously subaerial environment.

Inundation has been attributed to regional subsidence resulting in marine incursion during early opening of the Gulf of California (Lucchitta, 1979; Buising, 1990; Lucchitta et al., 2001; McDougall and Miranda-Martínez, 2014; Dorsey et al., 2018), or to filling of closed basins by first-arriving Colorado River water (Spencer and Patchett, 1997; House et al., 2008; Pearthree and House, 2014). A lacustrine origin is supported by Sr, O, and C isotopic evidence from basal carbonates (Spencer and Patchett, 1997; Poulson and John, 2003; Roskowski et al., 2010; Bright et al., 2018a), consistent maximum elevations of Bouse deposits within proposed paleolake basins (Spencer et al., 2013; Pearthree and House, 2014), and sedimentological evidence of floodwater influx derived from northern sources immediately preceding Bouse deposition in northern Mohave Valley (House et al., 2008; Pearthree and House, 2014). A marine origin is supported by the presence of several typically marine organisms (e.g., foraminifera, barnacles) represented by fossils from low elevations in the axis of the Blythe Basin, which is the southernmost of the Bouse basins (Smith, 1970; Todd, 1976; Crabtree, 1989; McDougall, 2008; McDougall and Miranda-Martínez, 2014; Dorsey et al., 2018), although a variety of factors allow for the possibility that all fauna lived in a brackish lacustrine environment (Bright et al., 2018b). In addition, some sedimentological features have been interpreted to indicate an estuarine origin (Buising, 1990; Turak, 2000; O’Connell et al., 2017).

In the lacustrine interpretation, deposition of the Bouse Formation resulted from the first arrival of Colorado River water and sediment to the Basin and Range province. It marks the initiation of a new river and incision of the modern Grand Canyon along its course (Spencer and Patchett,

2

1997; House et al., 2008). In the estuary interpretation, the Bouse Formation records a phase of subsidence associated with early rifting in the Gulf of California that is not obviously or necessarily related to Colorado River arrival and initiation of incision of the Grand Canyon, although the water body was subsequently filled with siliciclastic sediment supplied by the Colorado River (e.g., Dorsey et al., 2018). Furthermore, the presence of Bouse Formation of marine or estuarine origin at significant modern elevation would indicate the approximate amount of tectonic uplift since deposition, excluding eustatic sea-level fluctuations since the Early Pliocene. Proposed uplift has been tied to the timing of uplift of the Colorado Plateau and is a major reason for broad interest in the Bouse Formation.

Evidence of tides during Bouse Formation deposition

A study by O’Connell et al. (2017) of the southernmost area of Bouse Formation exposures identified several types of sedimentological features that were interpreted as representing the activity of tides, leading to the conclusion that carbonates and related sediments in the lower part of the Bouse Formation (the Marl 1 and barnacle hash facies of Homan (2014) and O’Connell et al. (2017)) were deposited in an estuary or tidal flat. Layered sequences of laminations or thin beds that are plausibly the result of daily (tropical) or twice daily (diurnal) tides were studied by O’Connell et al. (2017) using Fourier spectral analysis to identify periodicities associated with tidal cycles. One laminated carbonate sequence from the lower part (Marl 1) of the Bouse carbonate sequence (O’Connell et al., Fig. 3B), with 82 lamination couplets (paired light and dark layers), yielded two peaks with one that is twice the frequency of the other. These were interpreted as diurnal and semidiurnal tidal cycles. Periodicities of 4, 5, 6, 8, and 10 couplets per cycle for three other sequences were interpreted as incomplete records of 14-day spring-neap tidal cyclicity in tidal-flat environments.

As shown by Homan (2014, p. 72, section A20), there are two 4-6-m-thick marl units separated by ~2.5 m of bioclastic limestone in the Cibola area. A previous study of stratigraphically higher, thinly bedded marl unit (Marl 2) in the southern Bouse Formation had interpreted the laminations as annual varves (Homan, 2014). Only the lower marl (“Marl 1” of Homan, 2017) includes the laminated sediments interpreted by O’Connell et al. (2017) as tidalites.

Purpose of study

3

The original purpose of this study was to duplicate and potentially improve the results obtained by O’Connell et al. (2017) for laminated, possibly tidal sediments, but with a different data- collection procedure. Two of us (Spencer and Constenius) visited multiple exposures of Bouse Formation in the Cibola area of western Arizona and near Milpitas Wash directly to the west in southeastern California (Homan, 2014; Gootee et al., 2016) to locate thinly layered strata in the lower Bouse Formation that possibly represent sediments deposited in a tidally influenced environment, or alternatively, as annually deposited layers (varves). Data collection was done by photographing exposures from a distance of perhaps 10-20 cm and with a mm scale so that layer thickness could be determined. At one location (Site 6) samples were removed from the outcrop to optimize photographic resolution. We sampled sites regardless of past interpretations of specific depositional environments as our goal was to identify any laminated sediments plausibly related to tides and with a large number of laminations in a continuous sequence.

Data collection

Examination of photographs led to the determination that the laminations were microcrenulated due to surface exposure, laterally inconsistent in thickness, and too thin or too poorly defined over the many tens of laminations needed for precise measurement (Appendix A). At one location in lower Milpitas Wash west of the Colorado River, laminated marlstones were suitable for precise measurement. Marlstone near the top of the basal carbonate unit (the Marl 2 facies of Homan (2014) and O’Connell et al. (2017)) and just below the siliciclastic upper Bouse Formation consists of laminations that are both thicker (generally 1-2 mm) and better defined (site 6; Figures 1, 2). Furthermore, site 6 samples were readily removed from the fractured outcrop and photographed under ideal lighting conditions. Laminations from this site were especially visible because each consisted of a light-colored base grading upward into a dark- colored top, with a sharp upper bound overlain by light material of the overlying lamination (Fig. 3).

Digital photographs of site 6 samples (sample 6B) were placed in Adobe Illustrator™ and 1-mm tics were drawn using the scale that was photographed with the samples (Fig. 4). The 1 mm tics were then used to create evenly spaced 100-micron and 10-micron tics using the “Distribute objects” tools in the “Align” toolset. Magenta tics were then placed at the layer boundaries and the green scale tics were used to measure the distance between the magenta tics, yielding layer

4 thicknesses which are written at the site of each measurement in Figure 4. It was estimated during tic placement that uncertainty in measurements was ~0.01 – 0.02 mm. A total of 69 layers were measured in a continuous sequence. Thicknesses are listed and plotted in Appendix D (Excel file).

A polished thin section of a site 6B sample was photographed in plain (unpolarized) transmitted light with a petrographic microscope. The photographs are displayed in Appendix B with inferred lamination boundaries and internal boundaries between micrite and claystone indicated by brackets on the right side. Thicknesses were measured for about half the section. However, boundaries are indistinct and it is not apparent that laminations identified in hand sample can be correlated with laminations in thin section without adding specific markers before thin-section preparation.

Electron-microprobe analysis of the polished sections reveals that this rock is composed almost entirely of silt-sized calcite grains less than 10 μm in size (micrite) and clay (Appendix C, Figure 1) (note 200-micron scale bar at lower left of each image). Calcareous microlaminations appear as linear zones containing clusters of green and yellow calcite grains, while clay microlaminations appear as blue zones flecked with yellow and green grains primarily of calcite. The size and texture of the calcite grains is shown in the montage of images in Figure 2 of Appendix C (from a slightly different part of the polished section).

The remaining constituents of the sample are sparse ribbons (thin flakes distorted by compaction and, probably, hydration) and fragments of mica that appear on the potassium and magnesium maps, and a single relatively large grain of celestine (SrSO4) (Appendix C, Figure 1). Perhaps the most remarkable aspect of this sample is that there is little to no quartz. Note the almost complete lack of any silica signature on the Si map as indicated by the overall blue color (a quartz grain would look yellow or red if present). Examination of the entire polished section revealed rare silt-sized grains of quartz (<50 um).

Data analysis

A histogram of layer thickness shows a bimodal distribution, with all layers between ~0.7 and 3.0 mm thick (Fig. 5), which is generally similar to couplet-layer thickness measured by O’Connell et al. (2017, Data Repository) for strata east of the Colorado River. In the measured

5 site-6 strata, layer thickness was significantly greater for layers above ~layer 50 (Fig. 6). The 69 layer thicknesses were analyzed by Fourier spectral analysis (PAST software [Hammer et al., 2001] applied here by J. Bright) which yielded a simple periodogram showing identified dominant frequencies in the thickness sequence (Fig. 7). Only one peak, with a period of 60.4 layers, is statistically significant in this analysis. That peak is apparent in Figure 6 where the slight trough in the middle of the graph and the upturn at the left and, especially, the right, are reflected in a simple sine wave with a frequency of 60.4 layers per cycle. Only one cycle is represented by this power-spectrum peak, and we attribute no geologic significance to this peak because the record is not long enough to record multiple cycles.

All other power-spectrum peaks are below the level of statistical significance, with P values less than 0.05, which corresponds to less than 95% confidence that the periodicity is not due to random fluctuations in layer thickness (“white noise”). Specifically, with Fourier spectral analysis, the null hypothesis is that all spectral power is random noise, without any periodic signal that can be extracted from the complex power spectra. P values indicate the probability that a power-spectra peak is high enough to rule out the null hypothesis that the signal is in fact noise with no significance. Failure to disprove the null hypothesis at the 95% confidence level is interpreted here to indicate that there are no statistically significant periodicities in our data, and that we failed to identify any evidence for tides.

We note that periodograms do not become more accurate with more sampled points (layer thicknesses in this case) but rather that greater sampling (more layers) increases the range of frequencies identified with Fourier power spectra (Press et al., 1986, section 12.7 on “Power spectrum estimation using the FFT”). As shown in Figure 6, there is simply a trough in the data, but that trough is statistically significant. However, we don’t consider the trough to demonstrate cyclicity. We interpret this result to mean that one shouldn’t trust Fourier analysis for the power of low frequency cyclicity without considering the number of cycles actually sampled by the data.

To further evaluate the significance of periodic tidal signatures in Bouse strata, we applied Fourier spectral analysis to data represented in Figures 3B and 3C of O’Connell et al. (2017). As with Milpitas Wash sample 6B, we combined light and dark layers (dark on top) before Fourier analysis. Results (Figs. 8, 9) did not reveal any periodicities with >95% statistical confidence.

6

Thus, two previously studied locations from the lower Bouse carbonate sequence, interpreted to represent tidal deposition (O’Connell et al., 2017), and one new location from the upper Bouse carbonate sequence presented in this study, yield overall similar spectral periodograms that consistently lack statistically significant evidence of tidal periodicities.

The composition of laminated carbonates from the Milpitas Wash sample 6 site, with alternating layers of claystone and micrite, is consistent with an origin as annual varves reflecting seasonal changes, as inferred by Homan (2014). Tidal rhythmites are deposited over much shorter time intervals and are not generally associated with mineralogical changes in sediment type within laminations, whereas the type of alternation observed is common in seasonal or flood-pulse cycles that occur over longer time periods (Andy Cohen, University of Arizona, written communication, 2018). Tidal rhythmite intra-lamination variation is generally characterized by differences in sediment grain size, whereas varves are typically characterized by changes in sediment chemical and/or mineralogical composition that reflects changes in sediment delivery mechanism, biological activity, or other processes related to sediment sources (Cohen, 2003).

Conclusion

Field examination of thinly layered strata in the calcareous lower Bouse Formation in the Cibola area indicate that most material is not suitable for field photography and measurement of layer thicknesses for continuous sequences of tens of layers. Strata are either thinly laminated with inconsistent lateral thickness or have indistinct layer boundaries. Strata suitable for photographic evaluation were identified only in lower Milpitas Wash.

Fourier analysis of layer thickness variations in a continuous sequence of 69 layers of Bouse carbonate did not identify any statistically significant periodicities except a low frequency (long wavelength) signal that consists of only one cycle. Similarly, Fourier analysis of two samples analyzed by O’Connell et al. (2017) did not yield any statistically significant periodicities. Electron-microprobe analysis determined that laminations consist of alternating claystone and marl, which is consistent with annual changes in lake chemistry and sediment source rather than physical changes in sediment sorting and transport during tidal cycles. We conclude that all laminated micritic limestone in the southern Bouse Formation are varves, consistent with the interpretation that the Bouse Formation is lacustrine.

7

References cited

Bright, J., Cohen, A.S., and Starratt, S.W., 2018b, Distinguishing brackish lacustrine from brackish marine deposits in the stratigraphic record: A case study from the late Miocene and early Pliocene Bouse Formation, Arizona and California, USA: Earth Science Reviews, v. 185, p. 974-1003; https://doi.org/10.1016/j.earscirev.2018.08.011. Bright, J., Cohen, A.S., Dettman, D.L., and Pearthree, P.A., 2018a, Freshwater plumes and brackish lakes: Integrated microfossil and O-C-Sr isotopic evidence from the late Miocene and early Pliocene Bouse Formation (California-Arizona) supports a lake overflow model for the integration of the lower Colorado River corridor: Geosphere, v. 14; doi.org/10.1130/GES01610.1. Buising, A.V., 1990, The Bouse Formation and bracketing units, southeastern California and western Arizona: Implications for the evolution of the proto-Gulf of California and the lower Colorado River: Journal of Geophysical Research, v. 95, p. 20,111-20,132. Cohen, A.S., 2003, Paleolimnology: History and evolution of lake systems: Oxford University Press, 500 p. Crabtree, C.B., 1989, A new silverside of the genus Colpichthys (Atheriniformes: Atherinidae) from the Gulf of California, Mexico: Copeia, v. 1989, n. 3, p. 558-568. Dorsey, R.J., O’Connell, B., McDougall, K., Homan, M.B., 2018, Punctuated sediment discharge during early Pliocene birth of the Colorado River: Evidence from regional stratigraphy, sedimentology, and paleontology: Sedimentary Geology, v. 363, p. 1-33, https://doi.org/10.1016/j.sedgeo.2017.09.018. Gootee, B.F., Pearthree, P.A., House, P.K., Youberg, A., Spencer, J.E., and O'Connell, B., 2016, Geologic map of the Cibola area, La Paz County, Arizona, and Imperial County, California: Arizona Geological Survey Digital Map DGM-112, scale 1:24,000, with text. Hammer, Ø., Harper, D.A.T., and Ryan, P.D., 2001. Past: Paleontological statistics software package for education and data analysis: Palaeontologia Electronica, v. 4, n. 1, art. 4: 9 p.; http://palaeo- electronica.org/2001_1/past/issue1_01.htm. Homan, M.B., 2014, Sedimentology and stratigraphy of the Miocene-Pliocene Bouse Formation near Cibola, Arizona and Milpitas Wash, California: Implications for the early evolution of the Colorado River: Eugene, University of Oregon M.S. thesis, 116 p.

8

House, P.K., Pearthree, P.A., and Perkins, M.E., 2008, Stratigraphic evidence for the role of lake spillover in the inception of the lower Colorado River in southern Nevada and western Arizona, in Reheis, M.C., Hershler, R., and Miller, D.M., eds., Late Cenozoic drainage history of the southwestern Great Basin and lower Colorado River region: geologic and biotic perspectives: Geological Society of America Special Paper 439, p. 335-353; doi: 10.1130/2008.2439(15). Lucchitta, I., 1979, Late Cenozoic uplift of the southwestern Colorado Plateau and adjacent lower Colorado River region: Tectonophysics, v. 61, p. 63-95. Lucchitta, I., McDougall, K., Metzger, D.G., Morgan, P., Smith, G.R., and Chernoff, B., 2001, The Bouse Formation and post-Miocene uplift of the Colorado Plateau, in Young, R.A., and Spamer, E.E., eds., The Colorado River: Origin and evolution: Grand Canyon, Arizona, Grand Canyon Association Monograph 12, p. 173-178. McDougall, K., 2008, Late Neogene marine incursions and the ancestral Gulf of California, in Reheis, M.C., Hershler, R., and Miller, D.M., eds., Late Cenozoic drainage history of the southwestern Great Basin and lower Colorado River region: geologic and biotic perspectives: Geological Society of America Special Paper 439, p. 355-373; doi: 10.1130/2008.2439(16). McDougall, K., and Miranda-Martínez, A.Y., 2014, Evidence for a marine incursion along the lower Colorado River corridor: Geosphere, v. 10, n. 5, p. 842-869; doi:10.1130/GES00975.1 Metzger, D.G. and Loeltz, O.J., 1973, Geohydrology of the Needles area, Arizona, California, and Nevada: U.S. Geological Survey Professional Paper 486-J, 54 p. Metzger, D.G., Loeltz, O.J., and Irelan, B., 1973, Geohydrology of the Parker-Blythe-Cibola area, Arizona and California: U.S. Geological Survey Professional Paper 486-G, 130 p. O’Connell, B., Dorsey, R.J., and Humphreys, E.D., 2017, Tidal rhythmites in the southern Bouse Formation as evidence for post-Miocene uplift of the lower Colorado River corridor: Geology, v. 45, n. 2, p. 99-102; doi:10.1130/G38608.1. Pearthree, P.A., and House, P.K., 2014, Paleogeomorphology and evolution of the early Colorado River inferred from relationships in Mohave and Cottonwood valleys, Arizona, California, and Nevada: Geosphere, v. 10, n. 6, p. 1–22, doi:10.1130/GES00988.1. Poulson, S.R., and John, B.E., 2003, Stable isotope and trace element geochemistry of the basal Bouse Formation carbonate, southwestern United States: Implications for the Pliocene uplift history of the Colorado Plateau: Geological Society of America Bulletin, v. 115, n. 4, p. 434-444. Press, W.H., Flannery, B.P., Teukolsky, S.A., and Vetterling, 1986, Numerical recipes: The art of scientific computing: London, Cambridge University Press, 818 p.

9

Roskowski, J.A., Patchett, P.J., Spencer, J.E., Pearthree, P.A., Dettman, D.L., Faulds, J.E., and Reynolds, A.C., 2010, A late Miocene-early Pliocene chain of lakes fed by the Colorado River: Evidence from Sr, C, and O isotopes of the Bouse Formation and related units between Grand Canyon and the Gulf

of California: Geological Society of America Bulletin, v. 122, n. 9/10, p. 1625-1636; doi: 10.1130/B30186.1. Smith, P.B., 1970, New evidence for a Pliocene marine embayment along the lower Colorado River area, California and Arizona: Geological Society of America Bulletin, v. 81, p. 1411-1420. Spencer, J.E., and Patchett, P.J., 1997, Sr isotope evidence for a lacustrine origin for the upper Miocene to Pliocene Bouse Formation, lower Colorado River trough, and implications for timing of Colorado Plateau uplift: Geological Society of America Bulletin, v. 109, p. 767-778. Spencer, J.E., Patchett, P.J., Pearthree, P.A., House, P.K., Sarna-Wojcicki, A.M., Wan, E., Roskowski, J.A., and Faulds, J.E., 2013, Review and analysis of the age and origin of the Pliocene Bouse Formation, lower Colorado River Valley, southwestern USA: Geosphere, v. 9, n. 3, doi:10.1130/GES00896.1 Todd, T.N., 1976, Pliocene occurrence of the recent atherinid fish colpichthys regis in Arizona: Journal of Paleontology, v. 50, p. 462-466. Turak, J., 2000, Re-evaluation of the Miocene/Pliocene depositional history of the Bouse Formation, Colorado River trough, southern Basin and Range (CA, NV, and AZ): Laramie, University of Wyoming, unpublished M.S. thesis, 96 p.

10

Figures

Figure 1. Sample collection area, lower Milpitas Wash.

Figure 2. Milpitas Wash sample 6B collection site.

Figure 3. Site 6B hand sample. Stratigraphic top is to the upper right.

Figure 4. Upper and lower samples that together make up sample “Milpitas sample 6B”. Magenta tics were drawn at lamination boundaries and were used to measure lamination thickness. Sixty-nine lamination thicknesses were measured in a continuous sequence.

Figure 5. Histogram of layer thicknesses from Milpitas Wash sample 6B. Horizontal axis in millimeters, vertical axis in number of varves.

Figure 6. Varve thickness over stratigraphic sequence. Horizontal axis layer-sequence number moving up- section. Vertical axis in millimeters.

Figure 7. Periodogram produced by Fourier spectral analysis of layer thickness data from Milpitas Wash Bouse sample 6B. Horizontal lines represent P values for statistical significance, P=0.05 for lower line, P=0.01 for upper line.

Figure 8. Periodogram produced by Fourier spectral analysis of layer thickness data from O’Connell et al. (2017, Figure 3B) Bouse carbonates. Horizontal lines represent P values for statistical significance, P=0.05 for lower line, P=0.01 for upper line.

Figure 9. Periodogram produced by Fourier spectral analysis of layer thickness data from O’Connell et al. (2017, Figure 3C) Bouse carbonates. Horizontal lines represent P values for statistical significance, P=0.05 for lower line, P=0.01 for upper line. Appendix A: Field photographs of Bouse laminated marl and calcareous siltstone

Bouse Site 1A (lower Big Fault wash)

Site 2B (Big Fault wash, slot canyon)

Site 3A – 1 (O’Connell Fig. 3B approximate site, upper Cibola Wash)

Site 3A – 2 (O’Connell Fig. 3B approximate site, upper Cibola Wash)

Site 3A – 3 (O’Connell Fig. 3B approximate site, upper Cibola Wash)

Site 3B (O’Connell Fig. 3B approximate site, upper Cibola Wash)

Site 5 (Big Fault wash, slot canyon)

Site 5B (Big Fault wash – slot canyon)

Site 6A (Milpitas)

Site 7 (Milpitas)

Site 8 (Milpitas) 1

Site 8 (Milpitas) 2

UP Calcite

Clay

C MMMCCC BSE Ca

Mica Mica

K Mg

Quartz Celestine

Si Sr

Appendix C. Figure 1. Electron-microprobe compositional wavelength-dispersive spectroscopy (calcium, potassium, magnesium, silicon, strontium) and back-scattered electron (BSE) map images. Abbreviations: C - microlamination predominantly clay. M - microlamination predominantly micrite.

Appendix C, Figure 2. Electron microscope images (A, B, C) of microlayered marl and clay that make up ~1-3 mm laminations apparent in hand sample and in outcrop. Abbreviations: C - microlamination consists predominantly of clay. M - microlamination consists predominantly of micrite.