Elsevier

Sedimentary Geology

Volume 339, 15 June 2016, Pages 273-288
Sedimentary Geology

Quartz types, authigenic and detrital, in the Upper Cretaceous Eagle Ford Formation, South Texas, USA

https://doi.org/10.1016/j.sedgeo.2016.03.012Get rights and content

Highlights

  • The calcareous Eagle Ford Formation of South Texas contains both detrital and authigenic quartz.

  • Around 85 percent of total quartz in the Eagle Ford is interpreted as authigenic.

  • The dominant form of authigenic quartz is dark-luminescing matrix-dispersed microquartz.

  • Radiolaria, very poorly preserved, were the principal source of Si for quartz precipitation.

Abstract

Lithologic heterogeneity of the Eagle Ford Formation in South Texas arises from mixing of extrabasinal grains of siliciclastic composition with intrabasinal grain assemblages composed dominantly of marine carbonate with a lesser component of biosiliceous debris. Detrital quartz in particular is derived from both extrabasinal and intrabasinal sources, posing a challenge for the use of bulk compositional data for mudrock classification. Extrabasinal detrital quartz supplied along a major axis of siliciclastic influx, the Woodbine depositional system of East Texas, is reduced to a minor part of the grain assemblage in South Texas. Petrographic evidence and point-count results indicate that around 85 percent of total quartz in these rocks, equal to about 12.6 volume percent, is authigenic. Thus, significant quantities of authigenic silica are not restricted to siliceous mudrocks, but can be found in carbonate-rich mudrocks as well. Formerly opaline skeletons of radiolaria, the dominant source of silica for authigenic quartz precipitation, are only poorly preserved by replacements including calcite, dolomite, pyrite, and quartz. Dissolved silica released by dissolution of radiolarians, and perhaps also by volcanic glass dissolution is re-precipitated in a variety of forms, including matrix-dispersed microquartz cement, fillings within primary intragranular pores, and grain replacement of both calcareous and siliceous allochems. The mass balance of dissolved silica mobilized from radiolarians and other reactive silicates and the precipitation of authigenic quartz is uncertain because the initial volumes of now-dissolved detrital material versus the final volume of authigenic material (quartz and other authigenic silicates) cannot be determined with accuracy.

Introduction

Understanding of lithologic, textural, and chemical variation in mudrocks continues to be refined, but it is clear that mixing of extrabasinal and intrabasinal sources of detritus is an important control on mudrock heterogeneity (Aplin, A. C. and Macquaker, J. H. S., 2011, Macquaker, J. H. S. and Jones, C. R., 2002, Macquaker, J. H. S., et al., 2014, Milliken, K. L., 2014, Milliken, K. L., et al., 2012, Schieber, J. and Zimmerle, W., 1998). Extrabasinal detritus, primarily transported into basins via fluvial axes and to a lesser extent by eolian transport, includes material derived from crystalline rocks of the upper crust, older sedimentary rocks (including recycled organic matter), and weathering products. Intrabasinal detritus forms within the basin of the deposition as planktic or benthic grains, and consists of skeletal fragments, sediment aggregates, and organic matter.

Quartz is one of the dominant minerals in both the extrabasinal and intrabasinal fractions of muddy sediments (Blatt, H. and Schultz, D. J., 1976, Dean, W. E., et al., 1985, Potter, P. E., et al., 2005). Quartz also occurs in a variety of diagenetic forms in mudrocks (Isaacs, C. M., 1981, Kastner, M., et al., 1977, Keene, J. B. and Kastner, M., 1974, Milliken, K. L., 2013, Milliken, K. L. and Day-Stirrat, R. J., 2013, Milliken, K. L., et al., 2012). The specific form of authigenic quartz has significant implications for mechanical rock properties. Quartz in the form of grain-binding cement, as opposed to other diagenetic forms such as grain replacements, can impart lithification that causes brittle behavior in both natural and induced deformation (Milliken, 2013). Because mudrock quartz displays such variability in origin and form, measurement of bulk quartz content is of limited use for the determination of sediment source and rock property evolution (chemical and mechanical). Assessment of the character of quartz in mudrocks can only be accomplished by petrographic inspection.

Here, we examine the nature of quartz in the Eagle Ford Formation of South Texas. As will be shown in the following sections, much of the quartz in this unit occurs as clay-size crystals (< 4 μm) dispersed together with clay-size clay minerals and clay-size carbonate within the mudrock matrix. The generally small size of these crystals greatly limits what can be learned about them using conventional petrographic light microscopy. Discrimination of detrital3 versus authigenic origin of the quartz depends upon the use of field-emission SEM-based X-ray mapping and cathodoluminescence imaging. The principal finding of our investigation is that quartz in the South Texas Eagle Ford is primarily authigenic material that has formed in response to diagenetic recycling of opaline silica originally deposited as radiolarian debris. Authigenic microquartz can be an important constituent not only within siliceous mudrocks but also within carbonate-rich mudrocks such as those found in the Eagle Ford Formation.

Section snippets

Geologic setting

The Cenomanian–Turonian Eagle Ford Formation in the Maverick Basin of South Texas is a mixed siliciclastic/carbonate unit (Dawson, W. C., 2000, Donovan, A.D. and Staerker, T.S., 2010, Harbor, R., 2011, Lock, B. E., 2014, Tian, Y., et al., 2012, Workman, S. and Grammer, M., 2013). Deposition of the Eagle Ford Formation took place on a broad shelf during the maximum transgression of the western interior seaway (Donovan, A.D. and Staerker, T.S., 2010, Lowery, C. M., et al., 2014). Sediment sources

Sampling and methods

Forty-eight samples from the South Texas Eagle Ford Formation (Fig. 1; Table 1, Table 2) were thin-sectioned for this study. Samples were collected from one outcrop at Lozier Canyon (Donovan et al., 2012), and from four cores: a low-maturity well (Well 1; reported as Iona-1 well in Eldrett et al., 2014), a medium-maturity well (Well 2), and two high-maturity wells (Wells 3 and 4). The character of pores in this sample set (excluding the Lozier Canyon samples) is reported in Pommer and Milliken

Lithologic heterogeneity

Lithologies in the sample set are dominantly carls, mudrocks containing a predominance of calcareous bio-grains within the intrabasinal component of a grain assemblage containing greater than 25 percent intrabasinal material (classification of Milliken, 2014). Samples range from argillaceous carls (< 50% extrabasinal grains) to limestones (< 10 percent extrabasinal grains) (Fig. 2). Samples richest in extrabasinal grains contain minor silt-size detrital quartz, feldspar, and micas mixed with

Extrabasinal detrital quartz

Subangular to angular monocrystals of detrital quartz, mostly silt-size but ranging into the upper end of clay size, are present in almost all samples from the Eagle Ford Formation, but overall, are so minor in abundance that documentation of these grains requires active searching of the specimen. Quartz of extrabasinal origin shows angular shape and cathodoluminescence (CL) intensity and color that is bright and variable compared to authigenic quartz (Boggs, S., 2006, Sprunt, E. S., 1981,

Intrabasinal detrital quartz (former opaline grains)

Radiolarians are the dominant form of silica in the primary intrabasinal detrital assemblage (Fig. 5, Fig. 6, Fig. 7). Radiolarians are discriminated by their size, discrete spherical to oval shape, and the presence of spines (Fig. 5, Fig. 6). Compared to planktic foraminifers, calcified radiolarians generally lack a distinct wall as viewed in plane-polarized transmitted light. Sponge spicules have also been observed in the Eagle Ford Formation (Ergene, 2014).

The most abundant replacement of

Grain replacement

In addition to replaced radiolarians (described above), carbonate allochems such as foraminifers (Fig. 8), mollusks, and echinoderms are replaced by quartz. In some cases, the form of the quartz suggests replaced skeletal fragments, but the taxa of the replaced fossils are uncertain (Fig. 9). The cathodoluminescence of replacement quartz is dominantly dark (grayish) but red luminescing varieties are also observed (Fig. 10).

Cement

Quartz also takes the form of a cement in the Eagle Ford Formation,

Bulk quartz content

 The average bulk quartz percentage, based upon semi-quantitative XRD (Table 2), averages 14.7 weight percent for samples in Wells 2, 3, and 4. This bulk measurement incorporates quartz of detrital and authigenic origins.

Point count data (Table 3) reveal that detrital quartz of silt size (extrabasinal + intrabasinal), is a minor portion of the total quartz in this sample set. Qualitative observations in CL images further show that bright-luminescing detrital quartz characteristic of extrabasinal

General findings

Petrographic imaging reveals the presence of both detrital (particulate) and authigenic quartz in the Eagle Ford Formation. Authigenic quartz is by far the dominant form of quartz in these rocks as documented by qualitative image interpretation and comparison of petrographically assessed grain content and bulk XRD measurements.

Quartz in the carbonate-rich Eagle Ford Formation has surprising similarities in form to the more siliceous Mississippian Barnett Shale (Milliken, K. L., 2013, Milliken,

Conclusions

  • 1.

    Quartz in the Eagle Ford Formation, South Texas, includes detrital components of extrabasinal and intrabasinal origins, but is dominated (around 85 percent of total quartz) by authigenic quartz that takes several forms and composes about 12.6 percent of the rock volume.

  • 2.

    Authigenic quartz occurs as cements in primary intergranular and intragranular pores and as grain replacement.

  • 3.

    The dominant form of authigenic quartz is matrix-dispersed microquartz.

  • 4.

    The main source of Si for quartz was most likely

Acknowledgments

This study was supported by the Shell-University of Texas Unconventional Research (SUTUR) Project 3, “Characterization of Mudrocks by Quantitative Analysis of High-resolution SEM Images,” K. Milliken and N. Hayman, Co-PIs. SME was additionally supported by a Turkish National Petroleum Fellowship. KLM was able to visit the Lozier Canyon outcrop thanks to BP's kind permission for a field trip to their outcrop lease during the 2010 Hedberg Conference. We thank Patrick Smith for assistance and

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    1

    Current address: Turkish Petroleum-Research Center-Sedimentology and Reservoir Geology Department, Söğütözü Mahallesi, 2180, Cadde No. 10, 06530 Çankaya, Ankara, Turkey.

    2

    Current address: Middle East Technical University (METU), Department of Geological Engineering, METU Üniversiteler Mah. Dumlupınar Blv. No. 1, 06800 Çankaya, Ankara, Turkey.

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