Geosciences 433 – Lab 5 - 2010 Chemical sedimentary rocks (excluding evaporates and carbonates)

This week’s lab has no points associated with it. Rather, I’d like each of you to review the rocks described below in both hand specimen and thin-section (where available) and become familiar with each. Note that although I am not assigning points to this lab, you may see one or two of these rocks again on the midterm exam on Monday, November 15 and you will definitely see some of them again on the final exam in December.

Part I: Phosphorites: Review Boggs, p. 618-631 for an overview on the topic. Most of the rocks in this part of the lab were collected during my M.S. thesis. The published version of my thesis is the paper by Hendrix and Byers (2000), several copies of which are included in the lab. Be sure to look this paper over as you are examining the samples.

TLFS – Both this hand specimen and the thin section are representative of the basal contact of the type Phosphoria Formation which is a phosphatic hardground. Notice the irregularity of the hardground surface in the hand-specimen and thin-section and look for truncated grains on the surface in the thin-section.

95-HA-01: Basal part of Meade Peak phosphorite in Uinta Mountains. This sample is lithostratigraphically equivalent to the TLFS sample listed above. It is a classic transgressive lag deposit. Notice in both the hand specimen and the equivalent this- section BS7 the abundant phosphatic skeletal fragments, the phosphatic peloids and granules, and the well-rounded grains of quartz that suggest a significant history of transport.

Phos-Smok1; Phos-Smok2; Phosmon2: (thin-sections only) These samples were all collected from the main depocenter of the Phosphoria basin in southeastern Idaho. All are economic phosphorites that are currently being mined in the region. Compare the fabric of each of the three thin-section and note the various grain types. Pay special attention to the phosphatic nodules in sample Phosmon2.

HA11 – This hand-specimen and thin-section pair represent the heart of the phosphatic interval in the Uinta mountains transect. Notice the abundance of phosphatic peloids, the presence of larger granule-sized grains of phosphate, and the dark color that signifies an abundance of organic matter. In thin section, you will also see a variety of small skeletal fragments (mostly sponge spicules?) in the phosphatic peloids. For a second example of this peloidal phosphorite, have a look at sample HA9, collected just downsection of HA11 in the Meade Peak member.

HA29 – This sample of phosphorite (no thin-section) contains numerous steinkerns of phosphatic mud within a former mollusk shell. Notice the obvious biogenic shape of many of the larger grains. Thin-section samples DV7 and BS9 are similar.

RC29 and RC-? hand specimen: These two hand specimens, which are approximately equivalent to thin-section RC8 are good examples of a well-sorted peloidal phosphorite that is cemented with chert. These well-sorted phosphorites probably represent reworking and condensation of the less well sorted phosphorites formed in deeper water offshore (e.g., HA11 and HA29).

HA36 – This sample was collected at the top of the main phosphatic interval (Meade Peak member), where it undergoes a transition to overlying carbonate. In this part of the section, phosphorites are interbedded with dolomite and the section is pervasively burrowed. Notice in both the hand specimen and the thin-section the gray chert-replaced burrows. (These look like nodules, but they are probably burrows.) Notice also the abundant coarse grains of phosphatic sand, many of which are steinkerns. Lastly, notice that the chert is phosphate-free and the phosphatic layers appear to deform around the chert burrows somewhat, suggesting that the burrows may have become silicified during early diagenesis.

DV47b – hand specimen only. Check out the thin section HA 41 as an approximate equivalent to this hand sample. These samples are representative of the carbonate bank that prograded westward, into the Phosphoria basin following deposition of the main phosphatic phase (Meade Peak shale tongue). Note the abundant coarse sand size grains of phosphate. Many of these are steinkerns. Notice also the heavily bioturbated fabric of the rock, both the small wispy burrows a couple of mm across as well as the larger burrows that are 1 cm or so across.

BS49 – Like DV47b and HA41, this hand-specimen and thin-section pair is representative of the carbonate bank that prograded over the Meade Peak basin during Late Permian time. Notice the curved skeletal fragments, most of which are probably a phosphatic brachiopod called Orbiculoidea, as well as the finer-grained dark phosphatic sediment. The mottled fabric reflects abundant burrowing in this sample.

Part II: Siliceous sediments: review Boggs, pp. 583-598 for an overview

Mont (no thin-section): This is a classic diatomite from the Miocene Monterrey Formation of California. These diatomites, which are being mined along the central California coast, are generally interpreted as having been deposited in a series of silled offshore basins under conditions of intense oceanic upwelling. Notice the very delicate laminae preserved in the sample which are consistent with a lack of infauna due to the poorly oxygenated bottom conditions. The Monterrey formation is also one of the main petroleum source rocks in California, again the result of abundant organic matter deposited during the upwelling process.

94-FRAN: Jurassic Franciscan formation, Marin Headlands, central California coast. This is an excellent example of a radiolarian chert. Notice in thin-section that most of the radiolarian have been recrystallized – most of the delicate ornamentation associated with these sorts of planktonic organisms is lost during the recrystallization process.

Pp-McMm (no thin-section): Permian Phosphoria Formation, Rex Chert Member, McCarneys Mountain, southwestern Montana: This is a typical chert in hand specimen – very fine-grained, easily fractured.

Unk (no thin-section): This sample, of unknown age and locality, is an excellent example of chert nodules replacing a quartzose sandstone. Notice the irregular nature of the chert nodules and the fact that they weather out in positive relief, much like the chert-replaced burrows from the Phosphoria formation (see Hendrix and Byers, 2000 paper)

Part III: Banded Iron Formations: review Boggs, pp. 598-612 for an overview 05-PCBIF-01: Precambrian (Archean?) banded iron formation, Madison Valley, Montana: Typical example of a banded iron formation, in this case deformed. Notice the finely interbedded chert (yellow brown) and hematite (metallic luster).

UPM-9: Precambrian banded iron formation, near Ishpeming, Michigan: Typical example of banded iron formation from the upper peninsula of Michigan. These BIF’s are among the most important iron ores in North America. As with the other BIF, note the interbedded red chert and metallic hematite.