This page is a work in progress – images will be added as I wander through 30 years-worth of photos.
The photographs here are offered by myself and others who have kindly extended their permission. You are welcome to use these images but please credit them appropriately, either to myself or to the person indicated. Please contact me if you want to contribute useful images to this site.
NOTE: MARCH 31 2023: THIS PAGE IS UNDER MAJOR RE-CONSTRUCTION. Please be patient!
Franciscan Complex – sheared shale/broken formations/melanges
The Franciscan Complex (Franciscan) of Northern California is a jumble of crustal slices assembled as a jig-saw puzzle. It is famous for its many bodies composed of block-in-matrix rocks: olistostromes, broken formations, melanges and others. All appear similar and from an engineering point of view are largely synonymous: they are bimrocks. However, the geological origins and implications of the Franciscan bimrocks are the fodder for thousands of papers and books.
FYI: “olistostromes” are submarine landslide deposits – generally contorted sandstone blocks and shale matrix; “broken formations” are original sandstone/shale sequences, with the layers pulled apart and mangled; and, melanges are much like broken formations but contain “exotic” blocks which are lithologically, stratigraphically, chronologically, etc. inconsistent with the sandstones and shales. Exotics such as greenstones (metamorphosed oceanic crust basalts); eclogites and greenschists (highly metamorphosed rocks from great depths); limestones (shallow, warm water rocks); and others. The exotics can be extremely strong and troublesome in tunneling and excavations.
See also the PDF notes for additional images and observations
Gwna Melange – North Wales
In 1919, the British geologist Edward Greenly first described, as “autoclastic mélange”, the chaotic Gwna Melange of Anglesey, in North Wales. Mélange is a French word meaning “mixture”. My name “Medley” also means “mixture”. My heritage is Welsh. My interests in melange started when I first encountered it on a job site: the experience was so unsettling I went back to school and ended up researching (with Eric Lindquist and Prof. Richard Goodman) the engineering characterization of melanges and similar bimrocks.
So, of course I paid homage to Dr Greenly by visiting the Gwna Melange!.
See also the PDF notes for additional images and observations
Serpentinite bimrocks: serpentinite melanges and sheared serpentinites in Northern California
Serpentinites, also known as “serpentines” are metamorphosed basic to ultrabasic rocks such as gabbros, dunnites, and other oceanic plate rocks that are subducted. Serpentine minerals included asbestiform/asbestos minerals. They are called “serpentinites” because they are often greenish is color, with waxy, slick surfaces resembling the skin of snakes (serpents).
A sheared serpentinite is a mixture of sheared serpentinite matrix and dense, strong serpentinite blocks. Serpentinite melanges include blocks of other lithologies. Serpentinites are amongst the most troublesome rocks for geopractitioners.
Serpentinites are often associated with faults and pervasively sheared and are infamous for slope stability problems. Because of their association with landslides and asbestos, serpentinites are amongst the most troublesome rocks for geopractitioners.
Ironically, Serpentinite is the State Rock of California.
Geomorphology of Franciscan bimrocks – at hillside scale
Franciscan bimrocks are identifiable by their characteristic geomorphology (landforms), of which failing slopes and protruding blocks is typical. The presence of obvious blocks in the form of protruding rock masses, pinnacle and humps will alert the geopractitioner to inevitable buried blocks ranging from centimeters to possibly hundreds of meters in size, whose locations are unknown. The block-poor matrix rocks are generally weak, sometimes pervasively sheared and prone to landslides and erosion.
Please contact me if you want to contribute photos to this site but note that I only post useful images.
Use the resources freely, but please credit authors! All resources are provided on the condition that they be used for sole use, and for the non-commercial purpose of scientific and engineering research.