Sonyea Shales

Devonian Shale Facies - The Sonyea Group of New York

The Sonyea Group is a thin stratigraphic slice of the Devonian Catskill Delta Complex. We studied it because the internal stratigraphy was well understood, and prior studies had examined the interbedded sandstones and the paleoecology of infaunal and epifaunal brachiopods. In order to study how Paleozoic shale facies compares to that in the Proterozoic, this seemed a perfect framework for the study of lateral variability of shale facies along an onshore-offshore transect. The details of this study were published in JSR:

Schieber, J., 1999, Distribution and deposition of mudstone facies in the Upper Devonian Sonyea Group of New York. Journal of Sedimentary Research, v. 69, p. 909-925. download PDF file(left click on link and click on "Save as"...)

The Sonyea Group outcrops in upstate New York. It is sandwiched by two transgressive black shales, and thins dramatically from basin margin to basin center.

A scientific slide from an IU Shale Research study shows the location of the Sonyea Group in New York and Pennsylvania and a stratigraphic cross-section of its various shale formations.

On the basis of small scale sedimentary features (visible at hand-specimen and thin section scale), six onshore-offshore facies belts with multiple shale types were distinguished. Distinct sets of physical and biological parameters characterize each facies belt.

A scientific block diagram from the Sonyea Group Study shows a three-dimensional model of the Devonian basin, illustrating the various mudstone facies belts from a nearshore, high-energy environment to an offshore, deep-water setting.

Facies belt RM (red mudstones) comprises the coastal plain and consists of mostly red colored mudstones deposited in association with laterally migrating and aggrading rivers.

A scientific slide from the Sonyea Group Study shows a photograph of a coastal plain outcrop, with a close-up image of mudstone intervals and text describing fluvial sandstone bodies and mudstone features.

Mottled red mudstones with root traces are vertisols that reflect alternating wet-dry climate. Faintly banded and laminated-rooted red mudstones were the results of overbank deposition. Gray mudstones characterize oxbow lake infills. Mudstone conglomerates record energetic floods that reworked dried out mudcrusts.

A scientific diagram from the Sonyea Group Study shows the different types of Coastal Plain mudstones, with detailed drawings of their sedimentary features, including laminated, mottled, faintly banded, and conglomerated facies.

Nearshore mudstones commonly have silty/sandy laminae (SM, sandy mudstone facies), show deep scours filled with more mudstone, and are becoming increasingly bioturbated in offshore direction (BM, bioturbated mudstone facies).

A scientific slide from the Sonyea Group Study shows two photographs of a nearshore-offshore shale and mudstone outcrop, with labels highlighting features like storm sand deposits, deep scours, and ball and pillow structures.

Sandy mudstones show evidence of fall-out from rover plumes, wave and current reworking, moderate bioturbation (simple tubes), and sand event deposits (tempestites). In bioturbated mudstones evidence of physical sedimentation processes has been largely (but not entirely) obliterated. Strongly compacted Zoophycus burrows can impart a secondary "pseudo"-laminated texture.

A scientific diagram from the Sonyea Group Study shows the different facies of the nearshore-offshore transition, with detailed drawings of bioturbated mudstones and their sedimentary features, including burrows, laminae, and trace fossils.

Moving further basinwards, offshore sediment transport during storms built up a subaqueous platform with well developed HCS storm sands. The interbedded with gray shales/mudstones (facies belt GM) carry a combined imprint of storm deposition and fair weather bioturbation.

A scientific slide from the Sonyea Group Study shows two photographs of a rock outcrop representing the inner-outer platform environment, with a geologic hammer for scale in one image, and text describing interbedded sandstone and mudstone beds.

Severely bioturbated gray mudstones that only show traces of prior physical processes are interbedded (at the dm-scale) with mudstones that show variably preserved graded silt/clay couplets that are very similar to modern muddy storm deposits. These couplets are interpreted as reflection of more frequent weaker storms, whereas the HCS beds were produced by exceptionally strong storms.

A scientific slide from the Sonyea Group Study shows the characteristics of the Inner-Outer Platform mudstone, with detailed drawings of bioturbated facies and a magnified photograph of a bioturbation trace fossil.

Seaward of this storm produced platform, the bottom slope increased toward the basin center. Density currents transported fine sediment down this slope into the deepest portions of the basin where the most slowly deposited carbonaceous mudstones accumulated.

A scientific slide from the Sonyea Group Study shows the characteristics of the Slope-Basin facies, with a photograph of a steeply-dipping outcrop and a microscopic image of Bouma sequences and graded bedding.

In the slope facies prominent lamination was produced by deposition of fine-grained turbidites. At the basin floor bottom currents produced silt laminae in the organic matter-rich background sediment. Although bioturbation is minor, abundant benthic fecal pellets indicate that the very watery/soupy surface sediments were inhabited by worms, probably polychaetes with tolerance for low oxygen conditions.

A scientific slide from the Sonyea Group Study shows the characteristics of the Slope-Basin facies, with detailed drawings of bioturbation and fine laminations in the mudstone and a microscopic image of a fecal pellet.

Comparing with information gleaned from sandstone beds and paleoecology, we find that the mudstones not only match what we learned in prior studies, but allow considerable refinement of environmental assessment.

When compared to studies of unbioturbated Proterozoic shales, bioturbated shales provide much of the information on physical processes, and yield further information through interpretation of burrow features.

A presentation slide from the Sonyea Group Study summarizes the conclusions of the research, listing six key points about shale facies, basin analysis, and the informative role of bioturbation and benthic life.