Since the 1970s it has been recognized that cleavages, including crenulations, are often the result of pressure solution—a 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.

Source Byrne, 1994, Fig. 8.1a

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:

In the Delaware Water Gap area, well-developed slaty cleavage—i.e.,flow cleavage—occurs in an environment notably free of other criteria suggesting metamorphism. The obvious conclusion, that slatey cleavage is not necessarily a metamorphic phenomenon relating to folding at great depth, will be developed in this paper.

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't—at least initially. Numerous papers were published supporting and `debunking' his idea–or 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:

He didn't take on the paradigm and ultimately he was ignored. (Maxwell's fracture cleavage is pressure solution spaced cleavage. Note the theoretical inconsistency with strength envelopes.)

What is Greens Creek: A high grade polymetallic syngenetic strataform deposit of about 24mt. Mining has been active since 1986.

Some Enigmas:

• The deposit is highly deformed but the surrounding Triassic Hyd Group rocks are significantly less deformed. In the space of a kilometer footwall rocks grade from intensely deformed phyllites to volcanics rocks with primary features such as pillows.
• At the ore zone, intense foliation gives way to deformation that appears to be soft sediment with delicate depositional textures preserved. Deformation decreases up-section giving way to slaty sediments.
• “Recrystallized” remobilized massive sulfides are enclosed in relatively undeformed carbonate rocks.
• Clastic dikes cut the phyllites.
• Ductile deformation precedes brittle deformation of the same apparent age.
• A tensional vein set and guide to ore—described by one worker as diagenetic—is associated with intense compressive folding.
• The orebodies are complex in detail, but simple on a large scale.
• Some areas—especially near ore zones—resemble mélanges.
• Conglomerates with clasts of foliated footwall rocks, including serpentinites, occur at the ore horizon.
• The deposit is centered on stratabound ultramafic intrusive or volcanic rocks.

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.

What is AJ: The AJ is a roughly stratabound “mesothermal” gold deposit that produced 2.9 million ounces of gold between 1883 and 1944.

Some Enigmas:

• The deposit is located in intensely folded rocks at a particular stratigraphic horizon even though folding is not common in the area.
• Primary structures such as amygdaloidal pillow basalts are found below the deposit.
• Metamorphic grade varies independently of deformation across the deposit. The deposit surrounds stratabound mafic igneous rocks that have been described as sills but called flows in the past.
• The deposit is located at the hinge of a significant syncline that is enigmatic with respect to anticipated ore forming processes.
• Six stages of veining form a hugely complex system—the coincidence is overwhelming.
• Rock fabrics in the area of the mine range from fresh and undeformed, to phyllitic, to mélange textures, to schist, to slate. Some of these are within a short distance of each other.

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.

Folding adjacent to a mafic sill in the ore zone.

What is Treadwell: A stratabound and strataform “mesothermal” gold deposit that produced 3.2 million ounces of gold between 1881 and 1921.

Some Enigmas:

• Ore occurs in one “diorite” sill while nearby “diorites” remain barren.
• The ore-bearing Treadwell Diorite has a clastic texture.
• Ore is not necessarily associated with veins, but with ablitized or silicified zones.
• The deposit shows surprising variety of deformation. It is hosted in slate, overlain by greywacke and undeformed conglomerate and the footwall is phyllitic. Underlying Douglas Island Volcanics show relatively little deformation.

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.

Theoretical advances, field work, and experimental work of the the past 30 years all indicate what prior workers surmised for slaty cleavages—foliation 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 foliations—and the folding and clastic intrusives and other parts of this package—should 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.