Penecontemporaneous Foliation as a Continuum from Soft Sediment Deformation
Eric Twelker, P.O. Box 33873, Juneau, Alaska 99803
[This is an html version of a poster presented at the 2007 GSA meeting in Denver. A pdf version of the actual poster is here.]
Since the 1970s it has been recognized that cleavages, including crenulations, are often the result of pressure solutiona process favored in more porous and permeable rocks and not restricted to high pressure or temperature conditions. At the same time, it has been acknowledged that bedding plane foliations are commonly the result of diagenesis. And experimentalists have shown that pressure solution in a weak shear stress setting can result in wholesale recrystallization and create intense foliation and crenulation. The implications of this are that recrystallized and intensely foliated rocks may be formed during diagenesis in close proximity to soft and partly lithified sediment. In this poster, I review these developments and propose that in a shear stress scenario, such as sea floor collapse, a continuum of processes beginning at the sea floor with gravity induced slumping, subsurface soft sediment folds, subsurface sediment remobilization, various slips and folds in partially lithified rock, finally moving to rocks foliated, folded, crenulated and spaced cleaved by pressure solution processes.
I have applied the idea of a continuum to understand the structure at some ore deposits in Southeast, Alaska. The Greens Creek VMS deposit and the AJ and Treadwell gold deposits are centered on bulls-eyes of relatively intense apparently multiphase deformation surrounded by mostly undeformed rocks. Deformation is more intense lower in the stratigraphic sections. Within the deposits, fragile depositional textures and even fossils are found close to highly foliated and folded phyllites. Clastic dikes can cut the foliation. Apparently simultaneous ductile and brittle deformation are observed. Traditional structural-metamorphic explanations call for high P-T ductile deformation that is inconsistent with the observed textures. Interpretation as a continuum of soft sediment deformation to penecontemporaneous foliation in a shear stress regime resolves the inconsistencies and makes the deposits' structure easily understandable as the result of progressive deformation in a seafloor collapse.
An important implication of this theory is that measurement of structural features related to a collapse that is genetically related to ore deposits may be usable to model collapses and locate undiscovered ore deposits.
This poster asks readers to look at metamorphic textures in a different way. I ask your indulgence and seek your constructive comments.
In the 1970s . . . Petrologists recognized that cleavages, including crenulation cleavages, were mostly the result of pressure solutionthat constant volume metamorphic interpretations previously accepted without question, were wrong. What wasn't recognized were the implications of the observation: Unlike the constant volume theory, pressure solution is not dependent on high temperature or pressure to create cleavages. Instead, porosity and permeabilitythings that are much higher in near-surface lithifying rocks were key. Also a significant volume of rock would be lost to solution and this would have a structural impact.
flattened this phyllite
from Greens Creek by
more than 50%
. . . Constant volume deformation was an underlying but unspoken assumption in the debates [about the origin of cleavage]. . . . It now seems clear that the dominant mechanism for cleavage formation is [pressure solution] removal of the original host rock. Davis & Reynolds, 1996
In the 1980s .
regional bedding plane foliations were recognized as diagenetic. The thought was that compaction and
dewatering caused alignment of phyllosilicate minerals. The
ubiquity of these textures was such that formation by normally
directed stress didn't make sense. This was a good start, but the
idea wasn't carried forward. Many bedding plane foliations are
apparently too strong to be readily explained by simple
compaction and dewatering.
Source: Passchier & Trouw 2005
Fig. 4.6 FOV=1.8mm PPL
|Detrital micas may
be a starting
place for the diagenetic foliation,
but note how pressure solution
as evidenced by the opaque wisps
of insoluble mineralshas enhanced it.
Actually, crediting the 1990s for this observation may not be quite correct. Maxwell and those who followed the ideas expressed in his classic 1962 paper (see the blue box at the center of this poster) anticipated this.
In the 1990s . . . mélange studies demonstrated the multidimensional character of progressive penecontemporaneous deformation. Changes in deformation with depth and over the course of induration were documented and explained. The features observed include all of the things normally associated with conventional metamorphic deformation brittle and ductile deformation, intense foliations arising from pressure solutionand a few that weren't such as sediment remobilization features like clastic intrusives.
folding--including ptygmatic folding
of veins and beds is a mélange feature.
Source Byrne 1994, Figure 8.8a. Special thanks
to Tim Byrne for providing an
high resolution photo.
|The character of
sediment changes over
the course of progressive deformation.
Source Byrne, 1994, Fig. 8.1a
The distinction between tectonic deformation and soft sediment deformation is misplaced. One is a sediment state and the other is a force.
These studies did something else. They put all of the deformation features into the context of a surface down tectonic scheme. They turned some precepts on their heads too. For example, in mélanges ductile deformation might precede brittle deformation as rock lithified.
deformation in a mélange can
include multiple vein networks, breccias,
shearing, foliations, boudinage and folding.
From Vannucchi and Maltman, 2002, Figure 10.
In the 2000s . . . experimental work demonstrated shallow viscous flow by a pressure solution mechanism. The work, which was focused on mid-crustal faulting, found that rock analogues made up of phyllosilicates and minerals amenable to pressure solution, had shear strengths that were much less than expected. Bos & Spiers, 2002. A frictional-viscous flow occurred constrained by pressure solution.
and sometimes crenulations
were produced in the rotary shear experiments.
Photo source Niemeijer 2006.
|Bos and Spiers
proposed a model showing how
dissolution and recrystallization reform the rock.
Source Bos and Spiers, 2002, Figure 4.
suggested a radical change in the
strength envelopes for phyllosilicate-bearing rocks
a ductile shortcut from the brittle deformation of
Byerlee's law to the ductile flow laws.
Source: Bos & Spiers 2002 Figure 8c.
|The implication is
that old strength envelopes can't
be used to predict structures and fabric.
Modified from Bos & Spiers.
Experiments showed that foliation formed at lower shear velocities and more chaotic fabrics formed at higher velocities and where fluid pressures were higher. The paradigm wasn't working.
first . . .
cleavage . . .
A Proposed Scheme:
|Isn't this an old debate revisited . .
an idea debunked long ago . . .
In some ways it is and some it isn't. And any debunking isn't what it seems. In 1962, the eminent geologist, John C. Maxwell, was perplexed by clastic dikes and sills cutting slates. In a classic paper he expressed his purpose this way:
The idea of penecontemporaneous cleavage was (and is) a radical idea. But Maxwell's prominence was such that he couldn't be ignored. And he wasn'tat least initially. Numerous papers were published supporting and `debunking' his ideaor ideas. The notion of penecontemporaneous cleavage seemed to get lost in the details of his dewatering explanation. Among the more substantive of the criticisms was that dewatering couldn't create the effects, but pressure solution was a more likely candidate. (Geiser, 1975) Ultimately, interest waned and now dewatering and reorientation are noted as a reason for diagenetic bedding plane cleavage (Passchier & Trouw, 2005), post tectonic clastic dikes are written of, but pressure solution hasn't been taken up again. One purpose of this poster is to try to change that.
Maxwell's summary chart:
|A Hypothetical Look at Progressive Penecontemporaneous Deformation in a Seafloor Collapse:|
|Foliation is the first to form in the initial low shear stress environment. (These cartoons suggest the scale of a post-volcanic collapse. The same processes could apply in basinal-scale down-drops of Maxwell.)|
|Next, crenulation and spaced cleavage form. (Stretching is important, but is not considered here.) Early veins (not shown) will be folded by later progressive deformation.|
|As the collapse progresses, slumping and soft sediment folding occur near the surface. Higher stress and hydraulic over-pressuring at deeper levels can liquefy and brecciate rocks resulting in clastic intrusives that cut recently foliated rock. Where rock is sufficiently brittle, veins form. (A syngenetic massive sulfide deposit is shown in purple.)|
|Ultimately the deformation slows, stops and is buried. The deformed strata are preserved giving the appearance of a stratabound event.|
TESTING THE THEORY: I have used the idea of penecontemporaneous deformation and foliation to understand three ore deposits near Juneau, Alaska. In each of the examples discussed briefly below, if the deformation is understood as penecontemporaneous rather than as a result of later high pressure and temperature metamorphism, then the geologic interpretation is greatly simplified. Rock types that are enigmatic or disputed under conventional models are recognized as those associated with penecontemporaneous progressive deformation .
|TEST 1: Structure of
the Greens Creek Volcanogenic Massive Sulfide Deposit,
Admiralty Island, Alaska
What is Greens Creek: A high grade polymetallic syngenetic strataform deposit of about 24mt. Mining has been active since 1986.
The Conventional Explanation: Multiple phases of ductile regional metamorphism associated with continental accretion events have obliterated original textures and resulted in a complex refolding, much of it in a non-homogeneous pattern.* (Proffett, 2003) Most of the above enigmas remain unexplained.
The Progressive Penecontemporaneous Deformation Explanation: Greens Creek's deformation is localized areally and stratigraphically because it is the result of a seafloor collapse rather than regional metamorphism. The phyllitic textures are formed by pressure solution during diagenesis in a shearing or stretching regime. Hydrostatic over-pressuring associated with the collapse remobilized sulfides and the wall rockto form clastic intrusive conglomerates and apparent sulfide intrusives. Regional metamorphism did not destroy textures. The stratigraphic control of the deformation is the result of progressive deformation during lithification. All of the enigmas can be explained.
|One kind of ore horizon conglomerate|
|Pillow basalts in the Hyd Group|
|Mélange-like portions in the ore zone||Argillite caught in flowing sulfide|
|Folding resembles Maxwell's|
|TEST 2: Structure of
the AJ Gold Deposit, Juneau, Alaska
What is AJ: The AJ is a roughly stratabound mesothermal gold deposit that produced 2.9 million ounces of gold between 1883 and 1944.
The Conventional Explanation: The early-folded veins, the ore-centered-gabbros, and the metamorphic mineral assemblage as well as other structures all predate the ore-forming event. The surmise is that remote regional metamorphism associated with a 55Ma intrusive drove fluids to the syncline and mafic rocks where they ponded. (Redman et al., 1989) This speculation is based in significant part on Ar/Ar dates from hydrothermal sericite. The vein sets are attributed to simultaneous transpression despite indications that major transform movement did not begin until much later.
The Progressive Penecontemporaneous Deformation Explanation:
The syncline that forms the center of the ore zone and the locus of the mafic igneous rocks is actually a penecontemporaneous collapse feature. The vein sets are the result of progressive deformation during the collapse. The mafic igneous rocks are likewise associated with the collapse. The tensional regime is the simple result of that collapse as are the phyllitic foliations, melange-like textures and tight soft sediment style folding in the center of the deposit. Likewise the high variation in preservation of original textures is indicative of near surface deformation. Later regional metamorphism overprinted the area changing mineralogy and resetting radiometric dates.
Pillow basalt below the ore zone.
Parallel vein sets in the AJ glory hole.
Folding adjacent to a mafic sill in the ore zone.
|TEST 3: Structure of
the Treadwell Gold Deposit, Juneau, Alaska
What is Treadwell: A stratabound and strataform mesothermal gold deposit that produced 3.2 million ounces of gold between 1881 and 1921.
The Conventional Explanation: Fluids of derived from the same remote 55Ma metamorphic sources as A-J selectively replaced the 92Ma Treadwell Diorite while ignoring similar nearby rocks. (Fredericksen and Bressler, 1991)
The Progressive Penecontemporaneous Deformation Explanation: The slate, phyllitic texture, and folding are the result of seafloor collapse and associated progressive deformation. The undeformed greywacke and conglomerate are clastic intrusives. The orebody itself is probably a penecontemporaneous clastic intrusive derived from a system of great vertical extent and associated with a seafloor collapse.
|Mining the Treadwell diorite in 1906|
|The diorite is a clastic rock|
|Conglomerate overlying the ore is understood as a clastic intrusive|
|Cross-cutting clastic diorite from Mexican Pit #1 at Treadwell|
|Conclusions and Implications for
Theoretical advances, field work, and experimental work of the the past 30 years all indicate what prior workers surmised for slaty cleavagesfoliation can be, and often is, penecontemporaneous. Foliation by pressure solution processes can be the easiest path of deformation in phyllosilicate bearing rocks, thus foliation is very often the first textural expression of a progressive deformation. This is not just an odd happenstance, but something of major significance to interpretation of metamorphic textures everywhere. Textures commonly ascribed to certain metamorphic conditions may not be associated those conditions at all. They may be penecontemporaneous.
Application of the theory to the structure of three major mines in the Juneau area is indicative. When foliation is recognized as penecontemporaneous, enigmatic structures and lithologies become understandable. Folding is understandable as affecting soft or partially lithified rocks. Clastic intrusives seem a natural consequence rather than an unexplainable anomaly. Apparently stratabound deformation makes perfect sense. And, perhaps most importantly, numerous unexplainably coincidental deformation events can be explained as one progressive event. If Occam's Razor has any application at all in geology, then the idea of penecontemporaneous foliation as a continuum to and from soft sediment deformation needs to considered and understood.
The ideas expressed here have practical implications as well. It has long been recognized that many syngenetic ore deposits are associated with penecontemporaneous collapse. Syngenetitic massive sulfides and vein deposits are frequently found in the same district. But very often interpretations of metamorphic textures as being much later have prevented making connections. With recognition of foliations as penecontemporaneous, this should no longer be true.
What's more, those foliationsand the folding and clastic intrusives and other parts of this packageshould give indications of the direction, center, and intensity of the collapse. The colors on the cartoons on this poster give one first guess as to vectors to deposits located near the collapse center. Recognition of early foliation is but the first step in the much larger task in understanding these systems.
Bos, B., & C. J. Spiers (2002), Frictional-viscous flow of phyllosilicate-bearing fault rock: Microphysical model and implications for crustal strength profiles, J. Geophys. Res., 107(B2), 2028
Byrne, T. (1994). Sediment deformation, dewatering and diagenesis: illustrations from selected mélange zones. In: Maltman, A.J. (ed.) The Geological Deformation of Sediments, Chapman & Hall, London, 239260.
Davis, G.H. & Reynolds, S.J., (1996), Structural Geology of Rocks and Regions, (2nd ed.) Wiley.
Fredericksen, R.S., & Bressler, J, (1991) Progress Report for the Treadwell Project, Juneau, Alaska, 1990, Internal company report for Echo Bay Alaska, Inc.
Geiser, P.A., (1975) Slaty cleavage and the dewatering hypothesis. An. examination of some critical evidence. Geology 3, 717-720.
Maxwell, J. C., 1962, Origin of slaty and fracture cleavage in the Delaware Water Gap area, New Jersey and Pennsylvania: in Petrologic Studies: A Volume to Honor A. F. Buddington, Geological Society of America, p. 281-311.
Niemeijer, A.J., (2006) Effects of pressure solution and phyllosilicates on the slip and compaction behavior of crustal faults, PhD Dissertation Universiteit Utrecht, http://igitur-archive.library.uu.nl/dissertations/2006-0315-200011/
Passchier, C. W. & Trouw, R. A. J. (2005) Microtectonics (2nd ed.) Springer, Berlin.
Proffett, J. (2003), Geologic Structure of the Greens Creek Mine Area, Southeastern Alaska (unpublished draft Chapter 7 for proposed USGS Professional Paper).
Redman, E.C., Maas, K.M., Kurtak, J.M., & Miller, L.D., (1989) Bureau of Mines investigations in the Juneau Mining District, Alaska, 1984-1988, Bureau of Mines Special Publication.
Vannucchi, P. & Maltman, A.J., 2000. Insights into shallow-level processes of mountain building from the Northern Apennines, Italy. Journal of the Geological Society of London, v. 157, p. 155-120.
* Revisions made to this section at the request of John Proffett.