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Friday Field Foto #59: Patagonian deep-marine conglomerate deposits

July 18, 2008

This week’s Friday Field Foto heads back down to southern South America.

While doing my own PhD work down there, I had the opportunity to help out another student with his project on the underlying formation in the basin filling succession.

The Upper Cretaceous Cerro Toro Formation is well-known among sedimentologists for the spectacularly-exposed conglomerate member. These conglomerates were deposited in a marine basin in water depths of at least 1500 m (5000 ft) (based on foraminifera in overlying shale).

The conglomerates are turbidites and fill a ~5 km-wide channel belt that ran along the axis of the elongate foreland basin. Picture the submarine channel system running along the axis of Chile trench (althought that is in the subduction zone trench, whereas this would’ve been in the foredeep on the other side of the arc … but you get the idea).

In the photo above, note the two nearly-equal thickness packages – a lower one dominated by traction structures (the nearly-horizontal stratification) and some sandstone lenses, and an upper, more massive, package. The clasts are generally pebble to cobble sized and mostly Paleozoic metasedimentary rocks.

I’m second author on a recent paper in Sedimentology about this system. I am absolutely swamped right now and haven’t found the time to put together a good post about the study … stay tuned.

In the meantime, here’s the abstract:

Deep-water foreland basin deposits of the Cerro Toro Formation, Magallanes basin, Chile: architectural elements of a sinuous basin axial channel belt (Sedimentology, v. 55, doi: 10.1111/j/1365-3091.2007.00948 )

Coarse-grained deep-water strata of the Cerro Toro Formation in the Cordillera Manuel Señoret, southern Chile, represent the deposits of a major channel belt (4 to 8 km wide by >100 km long) that occupied the foredeep of the Magallanes basin during the Late Cretaceous. Channel belt deposits comprise a ca 400 m thick conglomeratic interval (informally named the ‘Lago Sofia Member’) encased in bathyal fine-grained units. Facies of the Lago Sofia Member include sandy matrix conglomerate (that show evidence of traction-dominated deposition and sedimentation from turbulent gravity flows), muddy matrix conglomerate (graded units interpreted as coarse-grained slurry-flow deposits) and massive sandstone beds (high-density turbidity current deposits). Interbedded sandstone and mudstone intervals are present locally, interpreted as inner levée deposits. The channel belt was characterized by a low sinuousity planform architecture, as inferred from outcrop mapping and extensive palaeocurrent measurements. Laterally adjacent to the Lago Sofia Member are interbedded mudstone and sandstone facies derived from gravity flows that spilled over the channel belt margin. A levée interpretation for these fine-grained units is based on several observations, which include: (i) palaeocurrent measurements that indicate flows diverged (50° to 100°) once they spilled over the confining channel margin; (ii) sandstone beds progressively thin, away from the channel belt margin; (iii) evidence that the eroded channel base was not very well indurated, including a stepped margin and injection of coarse-grained channel material into surrounding fine-grained units; and (iv) the presence of sedimentary features common to levées, including slumped units inferring depositional slopes dipping away from the channel margin, lenticular sandstone beds thinning distally from the channel margin, soft sediment deformation and climbing ripples. The tectonic setting and foredeep architecture influenced deposition in the axial channel belt. A significant downstream constriction of the channel belt is reflected by a transition from more tabular units to an internal architecture dominated by lenticular beds associated with a substantially increased degree of scour. Differential propagation of the fold-thrust belt from the west is speculated to have had a major control on basin, and subsequently channel, width. The confining influence of the basin slopes that paralleled the channel belt, as well as the likelihood that numerous conduits fed into the basin along the length of the active fold-thrust belt to the west, suggest that proximal–distal relationships observed from large channels in passive margin settings are not necessarily applicable to axial channels in elongate basins.

Happy Friday!

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10 Comments leave one →
  1. July 19, 2008 5:09 am

    Hey, a great turbiditic photo, as usual. This one is particularly interesting with the coarser grain size than I’m used to seeing.

    Your link to the article keeps telling me I need to accept session cookies, even though I think I’m accepting all cookies! Glad you published the abstract here. Look forward to your summary post.

  2. July 19, 2008 7:00 am

    Silver Fox … yeah, Blackwell has changed their site such that whenever I try and link it resets the page or something. I’ve had issues with their site the last couple of weeks.

  3. July 20, 2008 2:29 pm

    Blackwell was just bought by someone else, wasn’t it? Was it Elsevier? Anyway, the site has been FUBAR for a while.

  4. July 20, 2008 2:31 pm

    No it was Wiley that bought Blackwell. But they finally took over the Blackwell servers, and hilarity has ensued.

  5. Rich permalink
    July 30, 2008 5:58 am

    Hi Brian:

    Nice rocks! and nice work.

    Sounds like there was a great deal of sediment flux here. You mention Upper Cretaceous, do you know how much time is represented by the Lago Sofia Member? and has there been any work on sedimentation rates within the LS Member?.

    Would be interesting to attempt to build a geohistory and assign the levee sediments with the channel-fill. I’d bet there are some significant time gaps here. If so, they may be apparent in the levees.

    “turbidites”; “conglomerates” I’ll leave that to another day, when I’m feeling extremely pedant! :-)

    Cheers,

    Rich.

    ps. re photo above….. hard hats?….. anyone?….!! :-)

  6. July 30, 2008 6:43 am

    Rich … no, there is not much absolute age control in the basin, and none to get at sed rates within the Lago Sofia member itself. The microfossils in the under- and over-lying shale are slightly cooked (due to a nearby Miocene laccolith) making it difficult to recognize species in some cases.

    I would agree that the sed flux was likely high … but actually quantifying flux (volume sediment per unit time) will have to wait until we can somehow date the time a grain/clast was deposited (c’mon geochronologists!).

    These conglomerates were deposited in 1000-2000 m of water … so, while every single bed may not be a ‘turbidite’ sensu stricto (i.e., Bouma sequence), these beds do represent sedimentation several 10s to 100s of km from their entry point into the basin. And, yes, cobbles can be transported in suspension. :)

  7. Rich permalink
    October 21, 2008 11:57 pm

    Hi Brian:

    Indeed, cobbles can be transported in suspension. The reference to “conglomerates” was aimed at the use of the term. When does a conglomerate become a breccia? (50%?) How angular do the clasts have to be? (sub-rounded=conglomerate; sub-angular = breccia?). In this case, breccia being the sedimentological one, not the volcanic one!. Confusing?, may be.

    Can you think of another lithological descriptive term that is defined on both grain size and texture?

    Should one describe a granule grade siliclastic as a conglomerate? Well, to be precise I guess so, “granular conglomerate” may be.

    So my question is, why do we use “conglomerate”, when “rudite” may be a more suitable term, especially when used in conjunction with arenite?. In the same vein, why are argillaceous rocks rarely described as lutite?

    Lutite/arenite/rudite descriptors, also accommodates carbonates and siliciclastics, and a mix of the two quite well.

    On the other topic, have you seen a complete A-E bouma sequence in a single outcrop, or mapped along dip?

    Just a few random thoughts. May be a topic for the blog???

    Keep up the good work, always an enjoyable read.

    Regards

    Rich.

  8. October 22, 2008 6:44 am

    Rich, thanks for the comments and questions … I will try and answer and add my two cents in the coming days.

  9. October 23, 2008 2:29 pm

    Rich asks: “When does a conglomerate become a breccia? (50%?) How angular do the clasts have to be? (sub-rounded=conglomerate; sub-angular = breccia?).”

    I didn’t dig into my old textbooks, but my recollection is that to be a breccia they clasts should be quite angular. Your question is a good one, I’m not sure what the ‘cutoff’ is in angularity. I think sub-angular would still probably be called a conglomerate, but that would be something to look up.

    Rich asks: “Should one describe a granule grade siliclastic as a conglomerate? Well, to be precise I guess so, “granular conglomerate” may be.”

    Yes, I’ve seen the term ‘granular conglomerate’ used in papers. I’ve also seen the term ‘pebbly sandstone’ used when there is a distinct bimodality (pebbles and sand, but not much in between)

    Rich asks: “So my question is, why do we use “conglomerate”, when “rudite” may be a more suitable term, especially when used in conjunction with arenite?. In the same vein, why are argillaceous rocks rarely described as lutite? Lutite/arenite/rudite descriptors, also accommodates carbonates and siliciclastics, and a mix of the two quite well.”

    I’m not sure why that is not used. It seems to be more prevalent in the carbonate literature (which I don’t read often at all) but, as you point out, could be used generically. I guess it’s one of those things … interesting … I might do a bit more digging around.

    Rich asks: “On the other topic, have you seen a complete A-E bouma sequence in a single outcrop, or mapped along dip?”

    Oooh … very interesting (and fun) question. I think I’ll save that one for a blog post. Hopefully I can get to that soon.

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  1. Neoproterozoic turbidites in the Canadian Rockies « Clastic Detritus

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