Geology and Paleontogy of Stetson Bank
Richard Zingula, PhD., June 20, 2008
Retired Geologist, Volunteer, Recreational Diver
Many papers have been written about Stetson Bankfar too many to list here. The following is a short summary of geology and paleontology of the bank from my perspective. I made my first scuba dive on Stetson August 26, 1968 to photo and sample the bank for what was then Humble Oil & Refining Co. Since then I have taken numerous underwater photos (35mm slides), and collected a number of rock samples for geology and paleontology. All of that material is available to the Flower Garden Banks National Marine Sanctuary, if needed. Note that it might take me some time to locate all the materials!! Note also that I have seen only the western part of Stetson Bank, near where buoys have been placed over the years.
Stetson Bank is a distinctive "mountain" rising form the nearly flat sea floor in the outer part of the continental shelf in the northwestern part of the Gulf of Mexico. It is located in blocks 502 and 513 of the High Island South Addition area.
It is composed of nearly vertical beds, mostly of mudstone, with paper thin to 4+ inches thick beds of very hard siltstone. These rocks have been pushed up by a salt dome. Similar age beds are at a depth of approximately 10,000 feet below sea level in a well 14 miles southeast of Stetson.
Stetson, as is typical of many salt domes, is apparently composed of more than one block of upturned rock since some of the beds end within the area I have dived on. How many blocks may be present would require a lot of mapping and sampling.
Figure 1 shows (poorly), hard siltstone ridges that extend for some distance in the left photo. The right photo is a close-up that shows well that the siltstone resists erosion much better than the mudstone. Note that in that photo the mudstone has holes in it where the boring clam, Jouanettia quillingi, has been able to bore in. The siltstone is too hard for marine creatures to bore into it, but is a great place for them to adhere to and grow on as seen in Figure 4.
The siltstone shows cross-bedding in all samples I have examined. This is typical in turbidite deposits - i.e. sediment that has flowed rapidly down a sloping sea bottom then still moved where it came to rest. See Figures 2 and 3 for examples.
The siltstone samples were sawed with a diamond sawusually two cuts at right angles on each piece in order to get two different views of the patterns. It should be noted that all samples are quite hard, with the cement holding grains together mostly quartz15% hydrochloric acid caused only slight effervescence.
|FIGURE 1: Left - upturned beds of soft mudstone & hard siltstone, with overgrowth of sponges, etc. (6/22/86 #2-3). Right - Close-up showing differential weathering of siltstone and mudstone, with a long-spined urchin (Diadema) and numerous holes in the mudstone made by the boring clam (Jouanettia quillingi) (9/26/68). Photos: Dick Zingula
|FIGURE 2: Siltstone samples showing cross-bedding. Top - Sample collected 6/28/96 in Quadrat 4, 240 feet from morring in 75 feet of water at edge of drop-off. Bottom - Sample "A" collected at pinnacles near U-bolt 8/15/97. Both prints are x1. Photos: Dick Zingula
|FIGURE3: Siltstone samples showing cross-bedding. Top - Sample "C" collected 8/15/97 at pinnacles near U-bolt. Bottom - Sample "B" collected at same place and date. Both prints are x1. Photos: Dick Zingula
|FIGURE 4: Encrusting organisms growing on a siltstone block collected August 14, 1991. Small tubes are worms. Main encrusting growth is bryozoa (?) with five very small openings surrounding a slightly larger center one for each individual. Exact location this was collected is not given in my notes. Top print is x1.5, bottom is x4. Photos: Dick Zingula
I have not seen any large fossils. Age of the sampled rocks, where diagnostic microfossils are present is early Miocene age, i.e. 25 ± years old. Some of the samples do not contain age diagnostic microfossils. T.C. Huang, a micro-paleontologist with Exxon, examined four samples for coccoliths. Two were barren, and two contained early Miocene species. Some of the samples I examined for Foraminifera had only a few species, and those were not age diagnostic.
There is quite a bit of variation in fossil foraminiferal content from sample to sampleboth in abundance, and in species present. From 1968 to 1998 I collected, processed, and picked out Foraminifera from 35 mudstone samples. The "picked slides" are 60-hole cardboard slides, i.e., each cardboard slide of 1x3 inches in size has the center depressed and is printed with 60 numbered squares. That allows a micro-paleontologist to separate species into the numbered squares, or "holes". With many of the samples I have "picked", there are more than 60 species, requiring that two or more species be placed in each hole. Photographs of two 60-hole picked slides are shown in Figures 5 and 6. Although the magnification in the photos is not great enough to identify species, the photos do give some indication of species abundance. The species present are mostly Foraminifera, with a few ostracod species, and some tiny echinoid fragments.
One of my picked slides, with a representative microfauna is enclosed with this report for the Sanctuary. That sample was collected August 27, 1989; exact location was not recorded, only "70 feet, top of Stetson Bank". A list of species in each numbered square is given on page 7.
If the Sanctuary wishes to have more, and has room to store them, and has a use for them, that can be done. Also if the Sanctuary wants to make the picked slides, and/or washed residues available for some micro-paleontologist to do a further scientific study, that can be done, too.
There is a lot of variation in microfossil content between samples, suggesting that some of the mudstone with few forams came from a shallower depth originally and came down to deeper water as part of the turbidites, while samples with very abundant forams probably formed in place.
Many of the foram species present are not indicative of any precise water depth. However, many of the samples have species indicative of at least 3,000 feet of paleo water depth, and some may have been deposited in nearly 6,000 feet of water. Melonis pompilioides and Uvigerina hispida, especially the former, are such species.
Foram species such as Planularia venezuelana, Siphonina savisi, and Siphogenerina mayi (= S. fredsmithi in Gulf Coast paleo literature) are indicative of the early Miocene age.
Note that I have not had any samples processed and checked for palynomorphs, microfossil plant fragments.
Stetson Bank 60-hole slide contents - sample collected 8/27/89 at 70 feet
1-12 Recent age materials
13 Bathysiphon sp.
14 Ammodiscus, Haplophragmoides, Cyclammina
16 Textularia tatumi
17 Valvulina spinosa miocenica
18 Clavulina sp.
19 Several genera & species
20 Textularia miocenica
21 Textularia miocenica brevis (?)
22 Spiroloculina sp.
23 Melonis pompilioides
24 Uvigerina hispida
25 & 26 Lenticulina jeffersonensis
27 1 specimen of Robulus clericii; & miscellaneous Robulus spp.
28 Miscellaneous Robulus spp.
29 Planularia venezuelana
30 Saracenaria sp. (3 specimens); Marginulina sp. (1 specimen)
31 Marginulina (4 specimens, 3 species); Nodosaria, 3 species
32 Dentaline & Nodosaria species
33 Nodosaria species (right, ribbed is N. stainforthi)
35 left - Nodosaria sp.; right - Plectofrondicularia floridana
36 Plectofrondicularia vaughani (?)
37 Siphogenerina nodifera = S. texana
38 Siphogenerina mayi = S. fredsmithi
39 upper left - Uvigerina mexicana?
40 left - Bulimina pupoides
41 2 species of Bolivina
42 upper left - 1 specimen of Siphonina davisi, Gyroidina soldanii
44 left - Valvulineria sp.; right - Cassidulina subglobosa
45 Cibicides umbonatus?
46 Cibicides spp.
47 left - Cibicides sp; right - Globoquadrina sp.
48 Globorotalia mayeri?
50-59 various planktonic form genera and species
|FIGURE 5: Foraminifera in a 60-hole slide. There are more than one species in most holes since there were more than 60 fossil species in this sample. Sample was collected 50 yards west of anchorage on August 31, 1990. Prints are x4. Photos: Dick Zingula
|FIGURE 6: Foraminifera in a 60-hole slide with more than one species in each "hole, " in most cases due to more than 60 species present in the sample. Mudstone sample was collected near the mooring buoy on August 11, 1991. Prints are x4. Photos: Dick Zingula