Clastic Sedimentary Rocks and Sedimentary Melanges: Potential Naturally Occurring Asbestos Occurrences (Amphibole and Serpentine

JOHN WAKABAYASHI* California State University, Fresno, Department of Earth and Environmental Sciences, 2576 East San Ramon Avenue, Mailstop ST-24, Fresno, CA 93740-8039

Key Terms: NOA, Asbestos, Serpentinite, Glaucophane, Sandstones, Subduction Complexes

ABSTRACT

Petrography of mélange matrix and clastic sedimentary rocks in coastal California reveals the occurrence of detrital serpentine and detrital asbestiform sodic amphibole (glaucophane). Many sandstones of the Franciscan Complex have small amounts of detrital serpentine, with amounts of up to several percent in some cases. Detrital amphibole, including asbestiform glaucophane, is also present in some sandstones. Whereas rare sandstones have so much detrital glaucophane that they appear blue in hand specimen (up to nearly half of the rock volume), most glaucophane-bearing sandstones lack blue color, and the detrital glaucophane is not apparent in hand specimen. Most of the occurrences of detrital glaucophane are in blueschist facies sandstones, some of which also contain neoblastic (grew in place) glaucophane, but a notable exception is a widespread prehnite-pumpellyite facies unit that crops out primarily in Sonoma and Marin Counties. The detrital mineralogy of sandstones mirrors the block and matrix compositions of Franciscan mélanges that can be thought of as scaled-up equivalents of these clastic sedimentary rocks (mega-conglomerates/sedimentary breccias). Franciscan mélanges range from having a detrital siliciclastic to a detrital serpentinite matrix, and interfingering and gradation of the two matrix types is common. These findings suggest that clastic sedimentary rocks associated with current or past active orogenic settings elsewhere in the world may contain naturally occurring asbestos (NOA) even if the NOA component minerals are not visible in hand specimen.

INTRODUCTION

Whereas naturally occurring asbestos (NOA) was initially largely associated with serpentinite bodies, a growing number of studies (e.g., Metcalf and Buck,

*Corresponding author email: jwakabayashi@csufresno.edu

2015; Erskine and Bailey, 2018) have found occurrences of asbestiform minerals in a wide range of geologic settings, including various metamorphic rocks and hydrothermally altered plutonic rocks. Many rock bodies with asbestiform minerals are exposed in positions to potentially shed NOA detritus into various distributary systems that will ultimately lead to accumulation in various depocenters in terrestrial and marine environments. Quaternary alluvial deposits downstream of known NOA localities can be reasonably suspected of having the potential to contain NOA. In contrast, clastic sedimentary rocks that no longer have a direct spatial connection to their sources have thus far been overlooked as potentially containing NOA.

This contribution reports reconnaissance-level (petrography only) study of detrital serpentinite and detrital fibrous amphibole in clastic sedimentary rocks, including mélange matrix from the Franciscan Complex of coastal California. These findings have been previously reported in articles that emphasized interpretation of mélange origins (e.g., Wakabayashi, 2015, 2017a, 2019) rather than NOA.

GEOLOGIC SETTING

The Franciscan Complex makes up much of the bedrock of the California Coast Ranges (Figure 1) and is the world’s type subduction complex, formed by the transfer of rocks from the subducting to the upper plate (subduction-accretion), as the subduction thrust sporadically cuts into the top of the downgoing plate (Wakabayashi, 2015, 2017a). The upper part of the subducting plate (termed “ocean plate stratigraphy,” or OPS; Isozaki et al., 1990) consists of the uppermost part of the igneous oceanic crust (commonly basalt) overlain by pelagic sedimentary rocks (commonly chert), which is in turn overlain by clastic sedimentary rocks (mud rocks, sandstone, conglomerate) that represent trench fill. The clastic part of this triad is the most voluminous and it includes block-in-matrix units (mélanges), some of which have a serpentinite matrix (Wakabayashi, 2015, 2017a, 2019). The

Figure 1. General geology of northern and central California showing the Franciscan Complex, larger serpentinite bodies as well as the general localities (red unfilled circles) where detrital fibrous glaucophane and/or serpentinite were identified in this study. Note that the locations shown comprise multiple outcrops and thin sections. Map figure revised from Wakabayashi (2017a).

Franciscan Complex includes both serpentinite mélanges and intact slabs of serpentinite, but the largest bodies of serpentinite within the California Coast Ranges are associated with the Coast Range ophiolite or Great Valley Group (Wakabayashi, 2017b, 2017c).

DETRITAL SERPENTINITE AND AMPHIBOLE IN SANDSTONES AND MÉLANGE MATRIX

Detrital glaucophane and/or detrital serpentinite was identified by petrographic examination in over 100 thin sections from localities spanning a wide spatial range of units within the Franciscan Complex (some general localities shown on Figure 1). It is expected that these detrital components are present in a much larger part of the Franciscan that was not directly examined in this study.

Detrital serpentinite/serpentine has been found in sandstones of the Franciscan Complex in a wide range of units, with metamorphism of prehnite-pumpellyite to blueschist facies (Wakabayashi, 2015, 2017a) (Figure 2). A significant fraction of the detrital serpentinite in these sandstones appears to have been replaced by chlorite. Most Franciscan sandstones have small amounts (<1 percent) of detrital serpentinite, but less common examples (probably <5 percent of Franciscan sandstones) have amounts of up to about 5 percent. The higher concentrations of detrital serpentinite are associated with sandstones and conglomerates that are considered mélange matrix.

Sandstones and conglomerates with large amounts of detrital serpentinite (>50 percent) are present, but these have the appearance of serpentinite in hand specimen and outcrop, so the presence of serpentinite as a potential NOA source will be more easily recognized. The rocks composed of >50 percent detrital serpentinite have been mapped as serpentinite in previous studies and the detrital origins have been proposed on the basis of macro and micro textures, as well as stratigraphic relationships (e.g., Lockwood, 1971; Phipps, 1984; and Wakabayashi, 2012, 2015, 2017b, 2017c, 2019). Siliciclastic and detrital serpentinite mélange matrix in the Franciscan Complex has been interpreted to be interbedded, and they grade into one another on the basis of field and petrographic relationships (Wakabayashi, 2015, 2017a, 2017b). The sandstones and conglomerates with a large proportion of detrital serpentinite form serpentinite mélange in both the Franciscan Complex and the Great Valley Group (Wakabayashi, 2015, 2017a, 2017b, 2017c). Note that some otherwise siliciclastic sandstones of the basal Great Valley Group also contain detrital serpentinite (Figure 2). Some blueschist facies and prehnite-pumpellyite facies sandstones of the Franciscan contain detrital glaucophane that is commonly fibrous, as are neoblastic (metamorphic) overgrowths of glaucophane found only in the blueschist facies sandstones (Figure 2). Whereas detrital glaucophane is found mainly in blueschist facies sandstones, it is also common in a widely distributed unit in Sonoma County that comprises prehnite-pumpellyite facies conglomerates, sedimentary breccias, and sandstones (Wakabayashi, 2015). Whereas blueschist clasts in a conglomerate are easily recognized in outcrop, neoblastic (metamorphic grew-in-place) or detrital glaucophane (detrital blueschist clasts) is seldom recognizable in sandstone hand specimens owing to the relatively small grain size (commonly <0.5 mm) and the relatively small volumetric proportions of this material (generally <5 percent). An exception is a rare blue sandstone from El Cerrito composed of about 50 percent detrital and neoblastic glaucophane (Wakabayashi, 2017a). The fibrous glaucophane seen in detrital blueschist clasts in Franciscan sandstones is typical of the glaucophane seen in fine-grained blueschists of the Franciscan, present both as blocks-in-mélange and intact kilometer-scale bodies (Figure 2j–l) (compare to identical textures shown in Erskine and Bailey [2018]).

DISCUSSION AND CONCLUSIONS

The occurrence of detrital serpentinite and glaucophane in sandstones of the Franciscan Complex and Great Valley Group mirrors the matrix, clast, and block populations in associated mélanges (Wakabayashi, 2015, 2017a). Whereas serpentinite and blueschist are easily identified as pebble- or largersized clasts and blocks, these detrital components are not visually obvious in outcrops of associated sandstones. Although this study reports detrital serpentinite and glaucophane from the Franciscan Complex and Great Valley Group of coastal California, the general geologic relationships there suggest the likelihood of such detritus in younger sedimentary units that are derived from exposures of blueschist and serpentinite. The most likely setting for clastic sediment with a high potential for detrital glaucophane and serpentinite is Quaternary alluvium downstream of major exposures of serpentinite and blueschist. In such cases, however, the presence of detrital blueschist (and, thus, detrital glaucophane) and detrital serpentinite is easy to detect because of the abundance of gravel-sized and larger clasts in the alluvial deposits.

Miocene clastic rocks in the California Coast Ranges are sourced to variable degrees from the older Coast Range bedrock units such as the Franciscan Complex, Coast Range ophiolite, and Great Valley

Figure 2. Photomicrographs of detrital serpentinite and detrital glaucophane. Mineral abbreviations (for j): gln indicates glaucophane; hb, hornblende; jd, jadeite; qtz, quartz. a to h and k and l are presented as plane polarized and cross-polarized pairs, whereas i and j show cross-polarized and plane-polarized views alone, respectively. (a, b) A tremolite-chlorite-antigorite (meta-ultramafic) schist clast in prehnite pumpellyite facies pebbly sandstone from the cliffs of Black Sand Beach of the Marin Headlands. (c, d) Serpentinite clasts, one with chrome spinel, from a blueschist facies sandstone from Sunol Regional Wilderness, northern Diablo Range. (e, f, and i) From prehnite-pumpellyite facies pebbly sandstone from El Cerrito quarry, eastern San Francisco Bay area. (i) What appears to be cross-fiber chrysotile veins in the large serpentinite clast, whereas e and f show variable alteration of a serpentinite clast. (g, h) From basal Great Valley Group sandstone (zeolite facies or unmetamorphosed) at Chabot Reservoir, eastern San Francisco Bay area. (j) Detrital blueschist clasts with fibrous glaucophane as well as typical acicular glaucophane metamorphic (i.e., neoblastic, post-depositional) overgrowths on one of the clasts, from blueschist facies sandstone of El Cerrito quarry. The fibrous nature of glaucophane in fine-grained blueschists is typical in the Franciscan, as seen in k and l from a kilometer-scale blueschist body, the Rattlesnake Schist of O’Day (1974), near Cummings, northern California Coast Ranges. These textures are identical to those shown from blueschist from the Calaveras Dam area of the northern Diablo Range by Erskine and Bailey (2018). Photos a through d and j were presented with slightly different annotation in Wakabayashi (2017a), whereas the other photos are presented here for the first time. Photos a through d and i and j were taken at California State University, Fresno, whereas photos e through h and k and l were taken at Asbestos TEM Laboratories in Berkeley, CA.

Group (e.g., Graham et al., 1984) and may be expected to have detrital glaucophane and/or detrital serpentinite. For example, conglomerates of the Miocene Orinda Formation of the Contra Costa Group in the eastern San Francisco Bay region (Figure 1) contain abundant clasts of blueschist, although the source outcrops may be >40 km away (Wakabayashi, 1999).

In conclusion, the field and petrographic analysis presented in this article shows that clastic sedimentary rocks of the Franciscan Complex of California contain detrital serpentinite and detrital fibrous glaucophane. Assessment of the geology of coastal California suggests that such occurrences are widespread and that similar occurrences of detrital serpentinite and glaucophane are present in younger sedimentary units sourced from current and past exposures of blueschist and serpentinite. These findings have potential global implications for distribution of NOA. Clastic sedimentary rocks associated with current or past active orogenic settings may contain NOA even if the NOA component minerals are not visible in hand specimen.

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