|CADDISFLY (Trichoptera) larval cases
from Eocene Fossil Lake,
Fossil Butte National Monument
Mark A. Loewen1, V. Leroy Leggitt2 and H. Paul Buchheim3
1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84102
2Section of Geology, Department of Natural Sciences, Loma Linda University,Loma Linda, California 92350
Abstract—The Caddisfly (Trichoptera) larval cases from two sites in the Green River Formation of Eocene Fossil Basin are predominantly preserved as aggregates of calcareous tubes. The cases are tube shaped, slightly curved and generally lack sand grains or other particles in their case wall structure. Rare caddisfly larval cases from both sites show carbonate particles incorporated into the case structure. We believe that these caddisfly larval cases were constructed primarily of silk. The caddisfly larval cases are associated with lake-margin tufa, stromatolites and tufa-encrusted logs. This association illustrates the influence of metazoans in shaping the internal fabric of these Eocene lacustrine tufas.
Extant caddisflies are small insects closely related to moths and butterflies. Their unique larval stages are almost all aquatic. Caddisfly larvae can be divided into three categories of lifestyle related to their use of silk (Williams, 1989). The first category consists of those that are free living larvae that use silk strands to maintain position in flowing water. The second category consists of caddisflies that spin nets to catch food. The third group construct diverse portable cases made of silk, plant and/or mineral particles. These case-building caddisflies build either asymmetrical purse cases, bilateral tortoise cases, or tube cases (Wiggins, 1996).
Caddisfly larval cases have been recognized in several ancient lacustrine settings since Charles Lyell (1854) first reported fossilized cases from the Eocene of Auvergne, France. A. C. Peale first reported caddisfly larval cases from the Eocene Green River Formation during the Hayden expedition of 1877. Scudder (1878) believed these belonged to "some genus of Limnephilidae near Anabolia". Bradley (1924) described two additional types of caddisfly larval cases from the Green River Formation. All three types of caddisfly cases occur in rocks preserved from Lake Gosiute, east of Fossil Lake.
Since 1924, no new occurrences of caddisfly larval cases have been reported from the Green River Formation, despite intense paleontologic research. Researchers reported small tubes from two other sites in Lake Gosiute and interpreted them as "Oocardium tufa" (Bradley, 1974; Jensen and Buchheim, 1983). We have reinterpreted this "Oocardium tufa" as tufa that contains caddisfly larval cases (Leggitt et al., 1999).
Two new sites within Fossil Basin, adjacent to Fossil Butte National Monument (Figure 1), exhibit similar tubes to those previously interpreted as caddisfly larval cases from Lake Gosiute. These new sites represent the first reported occurrence of caddisfly larval cases from Eocene Fossil Lake.
GEOLOGIC SETTING AND STRATIGRAPHIC CONTEXT
Fossil Lake formed within Fossil Basin during the early Eocene (Figure 1). The Green River Formation of Fossil Basin is a lens of lacustrine limestone within the fluvial siliciclastics of the contemporaneous Wasatch Formation (Oriel and Tracey, 1970). Oriel and Tracey (1970) divided the Green River Formation of Fossil Basin into the Fossil Butte and Angelo members. Detailed stratigraphic correlation suggests that the assigned contacts between the two members occur at different stratigraphic horizons throughout the basin. Buchheim (1994) informally divided the Green River Formation in Fossil Basin into lower, middle and upper units of the Fossil Butte Member, based on lithologic characteristics. Due to ambiguity in Oriel and Tracey's divisions, we use the informal classification of Buchheim (Figure 1). The fossils in this report occur in the upper unit of the Fossil Butte Member of the Green River Formation.
The two sites with small tubes also contain stromatolites and tufa. They are located at the extreme edges of the Green River Formation in Fossil Basin (Figure 1) in rocks interpreted as nearshore-lacustrine facies (Leggitt, 1996; Loewen, 1999).
The first site (south shore) is located on a hill in the SW ¼ SW ¼ section 4, T. 19 N, R. 117 W near Warfield Springs. It is stratigraphically lower than the eastshore site. The sample was found as float from a regionally extensive bed of tufa-coated tree branches and logs (Figure 2). This bed consists of massive limey mudstone that grades basinward into facies with calcite pseudomorphs after saline minerals. The tufa-coated log unit at this locality is interpreted as the southern shoreline of Fossil Lake during deposition of the "maroon oil shale" unit of Loewen and Buchheim (1998).
Figure 1—Two sites with tubes interpreted as caddisfly larval sites are located in Fossil Basin (left). The stratigraphic relationships of the two sites to the Green River (stippled) and Wasatch formations (white) are depicted on the right (modified after Buchheim, 1994 and Oriel and Tracey, 1970).
The south shore site is interpreted as a freshwater margin of a saline alkaline lake (Loewen and Buchheim, 1998; Loewen, 1999). This freshwater margin in thesouth may have existed as an embayment fed by freshwater streams. This first site represents the extreme southeastern end of the lake during deposition of the maroon oil shale.
The second site (east shore) is located near the top of the south facing bluff in the NW ¼ NE ¼ section 12, T. 21 N, R. 117 W just north of westbound Highway 30 where it enters Fossil Basin. At this location, the unit is a kerogen-poor, completely bioturbated micrite with abundant ostracods and gastropods including Goniobasis. At the center of the basin this layer contains Magadi-type chert and evaporites (Loewen, 1999).
The east shore site is interpreted as a paleoshoreline where Fossil Lake lapped up onto the Eocene highs of the Oyster Ridge thrust belts (Loewen, 1999). This site represents the extreme eastern extent of the white marker bed. It pinches out a few meters to the east.
Figure 2—The stratigraphic position and lateral correlation of the two caddisfly larvae sites in marginal facies and their relationship to the basinal Type Section of the Fossil Butte Member.
Figure 3—A, Porous tufa with bark impressions forms the base of the south shore specimen. This is covered by a stromatolite layer (black arrows). Larval cases are located on top of the stromatolite layer. B, The forked end of the east shore specimen is covered with a finely laminated stromatolite coating and larval cases. Dashed lines indicate the surface on which the entire structure was deposited. Arrows indicate the finely laminated stromatolite layers.
Figure 4— A-D, Caddisfly larval cases from the south shore site. E-H, Caddisfly larval cases from the east site. A, Tufa-coated log with stromatolite layer and larval cases coating it from the south shore site. B, Stromatolite lamination and cross-section of the larval cases. C, Cross-section of a single larval case. D, Close-up of the curled particles coating a case. E, Caddisfly larval cases and the forked end of the log from the east shore site. F, Close-up of the stromatolite laminae coating the log. G, Close-up of the larval cases. H, Close-up of the curled particles coating an individual case.
DESCRIPTION OF THE TUBES
AND TUFA FROM BOTH SIDES
The south shore specimen has bark impressions on its basal surface. The bark impression is coated with a 43 mm thick layer of porous carbonate interpreted as tufa (Figure 3A, 4A). This tufa includes ostracods and pockets of finely laminated stromatolitic material (Figure 4B). The tufa is succeeded by a layer of finely-laminated columnar stromatolite 12 mm thick. This stromatolite layer is covered with small tubes (Figure 4C, 4D). The entire structure is coated with a 2 mm thick layer of finely-laminated stromatolite.
The specimen from the east shore consists of a roughly cylindrical, elongate shape resembling a log resting horizontally sub-parallel to the paleoshoreline. It is coated with finely laminated carbonate and surrounded by small tubes (Figure 3B, 4E). The cylindrical shape is interpreted as originally consisting of a log or tree branch. The wood later rotted and was replaced by bioturbated micrite. The log is 60 cm long with one end that is 24 cm wide and 11 cm high. The other end forks into two branches about 9 cm in diameter (Figures 3B and 4E), and is coated with a 3 _ 4 mm thick layer of finely-laminated stromatolite material (Figure 4F). The stromatolite surface is covered with small tubes (Figure 4G). Carbonate detritus and ostracods form the matrix around the cases. The entire structure is coated on the top surface with a 3 mm thick layer of finely-laminated stromatolite.
The small tubes at both sites consist of cylindrical, slightly curved elongate structures. The tubes are slightly tapered. They range from about 15 to 20 mm long and 1.7 to 2.0 mm in internal diameter. External diameter ranges from 1.9 to 2.2 mm. In cross section the tube walls exhibit rare angular and curved carbonate fragments from 0.1 to 0.2 mm in diameter comprising the tube wall (Figure 3D, 3H). The tube walls also contain rare, angular quartz grains about 0.1 mm in diameter. The tubes are coated inside and out by finely laminated carbonate.
The slightly curved tubes from both sites are interpreted as caddisfly larval cases. They are similar to other reported caddisfly larval cases from the Green River Formation (Scudder, 1878; Bradley, 1924). The larval cases from Fossil Basin may have been primarily constructed from silk because case-building particles are rarely observed. Some of the cases contain small carbonate grains, which have been incorporated into the case structure. These curved carbonate grains may be small clastic fragments or pieces of stromatolites and tufa that the larvae harvested from their surroundings. Similar quarrying behavior has been noted in modern caddisflies (Drysdale, 1999). The finely laminated carbonate coating the tubes inside and out was probably deposited after the cases were abandoned.
The logs with tubes coating them from both sites are coated with either porous tufa or a layer of finely laminated stromatolite material. This suggests that the log was submerged in place for a period of time. Caddisfly larvae subsequently colonized this surface. The aggregation of cases formed a chaotic, rigid latticework that may have acted as a baffle that collected small carbonate particles between the cases. Ostracods occur in the matrix surrounding the cases in both specimens.
Today, caddisfly larvae are associated with shallow, well-lighted, well-oxygenated, nearshore-lacustrine and fluvial environments (Mackay and Williams, 1979). They have been used as indicators of nearshore-lacustrine paleoenvironments in the Green River Formation (Bradley, 1928). The tubes interpreted as caddisfly larval cases from the two sites in Fossil Basin are consistent with the marginal lacustrine interpretation of these two sites (Loewen, 1999).
The co-occurrence of caddisflies with tufa at these sites suggests a link between metazoan insect larvae and the tufa- and stromatolite-building processes. It is likely that the caddisfly populations directly contributed to megascopic tufa fabric formation and were strongly involved in reshaping the micro- and macroscopic fabric of this tufa. Recent studies have shown the importance of aquatic insect larvae, including Trichoptera larvae, in the development and alteration of some modern tufa fabrics (Humphreys et al., 1995; Drysdale, 1999).
A research grant from Fossil Butte National Monument and financial support from Loma Linda University supported this research. The rangers and staff of Fossil Butte National Monument provided encouragement and logistical support, which has proved invaluable to several of our ongoing projects. In particular we would like to thank David McGinnis, Vince Santucci, and Arvid Aase of Fossil Butte National Monument.
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