WHERE IS THE CHEMOCLINE? PART 3: PYRITE FOSSIL BEDS
Welcome to Part 3 of our discussion on the position of the chemocline in ancient basins. In Part 1, which you can read HERE, we introduced the issue at hand: When looking at ancient mudrocks, can we determine where the chemocline resided? That is, how far into the water column did bottom water conditions extend? In Part 2, we discussed pyrite framboid size distribution, and how this measure can tell us if the chemocline was at or below the sediment-water interface, or in the water column. You can read all about that HERE. For this post, I’d like to look at something quite different. I’d like to focus on observations myself and others have made while collecting fossils in grey shale units. Specifically, I would like to discuss the occurrence of pyrite fossil beds in Hamilton Group strata overlying the Marcellus Shale, how they form, observations regarding their sedimentology and stratigraphic associations, and their faunal assemblages.
Pyrite beds are an aspect of sedimentary successions I am just beginning to study. Key references I have found invaluable are in the early work Carl Brett and Gordon Baird, and specifically the reference by Dick and Brett, 1986: “Petrology, Taphonomy and Sedimentary Environments of Pyritic Fossil Beds from the Hamilton Group (Middle Devonian) of Western New York” which appeared in the New York State Museum Bulletin Number 457 titled Dynamic Stratigraphy and Depositional Environments of the Hamilton Group (Middle Devonian) in New York State (Part 1).
I wouldn’t say that pyrite beds are common, but they certainly are not rare. There are numerous descriptions of their occurrence from Paleozoic and Mesozoic strata. Dick and Brett note they are described from the Devonian Silica Shale, Mississippian Fayetteville Shale, and the Jurassic Posidonienschiefer and Oxford Clay for example. I have seen examples from the Devonian Arkona Shale, Mississippian New Providence Shale, and I have collected from pyrite beds in the Cretaceous Del Rio Formation (see ammonites and gastropods in this picture):
The process of pyritization in these beds is no different from sedimentary pyrite formation that we have discussed previously. Reactive iron combines with hydrogen sulfide under anoxic conditions to form pyrite. The difference here, as oppose to organic-rich muds, is that hydrogen sulfide, and not iron, is the limiting factor on pyrite production. The absence of abundant organic matter in the sediment precludes widespread bacterially mediated sulfate reduction (BSR). Instead, this process occurs within decaying organic matter largely confined to microanoxic environments created within enclosed cavities such as worm burrows, infaunal bivalves, articulated brachiopods, enrolled trilobites, and cephalopod chambers. Hydrogen sulfide generated BSR in the decaying material reacts with reactive iron in the surrounding sediment to infill these cavities, and producing pyrite stienkerns.
Indeed, Dick and Brett note that epifaunal bivalves which tend to splay open upon death, disarticulated brachiopod shells, and trilobite fragments are rarely pyritized.
Numerous pyrite beds occur in the upper divisions (Ludlowville and Moscow Formations) of the Hamilton Group in the Appalachian Basin. To date, as many as eight distinct beds have been identified. It is worth noting that this discussion does not report on the occurrence of detrital pyrite lags (sometimes referred to as beds) such as the Leicester Pyrite, which occur throughout the Middle and Upper Devonian Shale succession. The beds discussed herein are not detrital in nature. Rather, they comprise a certain thickness of dark-grey shale rich in pyrite nodules, pyritized worm burrows, and pyritic fossil stienkerns, many of which show evidence of further pyrite overgrowths. Dick and Brett note that these beds normally occur in transitional facies between dark-grey and black shale facies which often have sedimentological and geochemical evidence of widespread anoxia and sparse nektonic fossils, and light-grey calcareous facies with abundant benthic and nektonic faunal assemblages.
The faunal assemblages of pyrite beds are notably distinct from those of adjacent light-grey shale, consisting of a few small species of brachiopods, nuculoid bivalves, an assortment of gastropods, the trilobites Greenops boothi and Eldregeops rana, the orthocone nautiloids Michelinoceras sp. and Spyroceras sp., and the goniatite Tornoceras uniangulare. This would suggest a relationship between pyrite bed facies and faunal communities. Here is an example of fauna I have recovered from perhaps the most well studied of these pyrite beds, The Alden Pyrite:
Absent are the large corals and brachiopods and associated fauna found in adjacent beds. What’s more, the faunal assemblages of pyrite beds are consistently “diminutive”. For example, look at this comparison of Ambocoelia recovered from the Penn Dixie Pyrite Bed (bottom row) compared to those recovered from immediately overlying light-grey shale (top row):
The dark-grey mudstone, faunal assemblages and their diminutive size all indicate that pyrite beds of the Hamilton Group accumulated under dysoxic (O2 < 0.5 ml/L) conditions. Stressed conditions associated with a low oxygen environment not only shifted the ecological makeup of benthic communities, but also potentially stunted their growth. What I find very interesting is that this scenario is also evident in the nektonic community. Indeed, we can rightly classify the cephalopod communities as diminutive. Here is an example of typical Tornoceras uniangulare collected from the Alden Pyrite compared to one I collected from the Moscow Formation in Lebanon, NY:
Further, I sieved Alden Pyrite samples I collected and noted the occurrence of Tornoceras uniangulare relative to sieve size. Analysis of over 300 goniatites indicates that while large examples (greater than 12.5 mm/0.5 in) occur, they are quite uncommon, and the population is mostly very small (> 90% are 2-6 mm in diameter). This data strongly suggests that benthic oxygen conditions extended up into the water column enough that it also affected the occurrence and growth of nektonic organisms.
With few exceptions benthic and nektonic fauna are largely absent, or occur in low diversity in black shales. For example, The Cherry Valley Limestone contains wonderful examples of the large goniatite Agoniatites vanuxemi and straight-shelled cephalopods. They do not appear in the underlying black shale of the Union Springs, and are absent from black shale facies of the overlying Oatka Creek. I have found goniatites in black shale of the Upper Devonian Rhinestreet Shale (see picture below), but they are confined to a single bed (The Rhinestreet is > 54 m/150 ft thick). Again, this isn’t to say that nektonic communities don’t exist, large placoderm fish fossils are found in black shale, but they are not ubiquitous throughout the stratigraphic section.
To summarize, light-grey shale in the Hamilton Group exhibit abundant and diverse infaunal and epifaunal benthic communities which co-occur with large nektonic cephalopods. Dark-grey shale-hosted pyrite beds represent dysoxic facies with stressed conditions, and comprise a lower diversity, diminutive fauna in both the benthic and nektonic community. Finally, black shale with geochemical indicators of anoxic to euxinic bottom waters are characterized by very sparse to absent benthic and nektonic communities. Such observations suggest that, at a coarse scale, bottom water conditions in the Hamilton Group are mirrored in some (significant) portion of the overlying water column.
Next week I’d like to discuss the geochemistry of black shales, both work that myself and colleagues have done, and work recently published by others. Thanks for reading. This post was a bit delayed because I took advantage of stream and weather conditions to get out in the field earlier this week. There is a short video of what I was up to on Applied Stratigraphix YouTube channel, and you can watch it HERE. Some fossil collecting was involved!