Master of Science

thesisCaddisfly larvae construct underwater protective cases using surrounding materials, providing information on environmental conditions in both modern and ancient systems. Microbial bioherms associated with caddisfly cases are found in the Berriassian-Hauterivian (~140-130 Ma) Shinekhudag Formation of Mongolia and provide new insights into aspects of lacustrine paleoecosystems and paleoenvironments. This formation contains the earliest record of plant-armored caddisfly cases and a rare occurrence of microbial-caddisfly association from the Mesozoic. The bioherms are investigated within the context of stratigraphic correlations, depositional environment interpretations, and basin-evolution models of the sedimentary fill. The bioherms form 0.5-2.0 m diameter mound-shaped bodies and are concentrated within a single, oil shale-bound stratigraphic interval. Each bioherm is composed of up to 40% caddisfly cases along with millimeter-scale, laminated stromatolites. Petrographic analyses reveal these bioherms are composed of non-systematic associations of columnar and oncoidal microbialites, constructed around colonies of caddisfly cases. The cases are straight to curved, slightly tapered, tube-shaped, with a progressively increasing length and width trend (7-21 mm by 1.5-2.5 mm). Despite these variations, the case architectures reveal similar construction materials; the armor is dominated by plant fragments, ostracod valves, carbonate rock fragments, and rare mica and feldspar grains. The bioherms contain various allochems including ooids, ostracods, plant fragments, rare gastropods, feldspar grains bound in micritic matrices, and carbonate dominated cements. The combination of microbial-caddisfly association, plant fragment case armors, and ooids/oncoids indicates a shallow, littoral lake setting. Stratigraphic juxtaposition of nearshore bioherms and the bounding distal oil-shale facies suggests that the bioherms developed in an underfilled lake basin, resulting from rapid lake desiccation. Lake chemistry is believed to have been relatively alkaline, saline to hypersaline, and rich in Ca, Mg, and HCO3 ions. Through analyzing bioherm characteristics, caddisfly case architecture, carbonate microfacies, and stratigraphic variability, we infer larger-scale processes that controlled basin development during their formation

Microbial-caddisfly bioherm association from the Lower Cretaceous Shinekhudag Formation, Mongolia: Earliest record of plant armoring in fossil caddisfly cases

Caddisfly larvae construct underwater protective cases using surrounding materials, thus providing information on environmental conditions in both modern and ancient systems. Microbial bioherms associated with caddisfly cases are found in the Berriassian-Hauterivian (similar to 140-130 Ma) Shinekhudag Formation of Mongolia, and yield new insights into aspects of lacustrine paleoecosystems and paleoenvironments. This formation contains the earliest record of plant-armored caddisfly cases and a rare occurrence of microbial-caddisfly association from the Mesozoic. The bioherms are investigated within the context of stratigraphic correlations, depositional environment interpretations, and basin-evolution models of the sedimentary fill. The bioherms form 0.5-2.0 m diameter mound-shaped bodies and are concentrated within a single, oil shale-bound stratigraphic interval. Each bioherm is composed of up to 40% caddisfly cases along with stromatolites of millimeter-scale, micritic laminations. Petrographic analyses reveal these bioherms are composed of non-systematic associations of columnar and oncoidal microbialites, constructed around colonies of caddisfly cases. The cases are straight to curved, slightly tapered, and tube-shaped, with a progressively increasing length and width trend (7-21 mm by 1.5-2.5 mm). Despite these variations, the case architectures reveal similar construction materials; the particles used for cases are dominated by plant fragments, ostracod valves, carbonate rocks, and rare mica and feldspar grains. Allochems within the bioherms include ooids, ostracods, plant fragments, rare gastropods, feldspar grains bound in micritic matrices, and are consolidated by carbonate dominated cements. The combination of microbial-caddisfly association, plant fragment case particles, and ooids/oncoids are indicative of a shallow, littoral lake setting. Stratigraphic juxtaposition of nearshore bioherms and the bounding distal oil-shale facies suggests that the bioherms developed in an underfilled lake basin, resulting from an abrupt and short-lived lake desiccation event. Lake chemistry is believed to have been relatively alkaline, saline to hypersaline, and rich in Ca, Mg, and HCO3 ions. Through analyzing bioherm characteristics, caddisfly case architecture, carbonate microfacies, and stratigraphic variability, we infer larger-scale processes that controlled basin development during their formation.Fulbright AssociationOpen access journal.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

List of hand samples and classifications.

<p>List of hand samples and classifications.</p

Regional stratigraphic correlation of Late Mesozoic of Mongolia (modified after Barry, 1999; Graham et al., 2001; Johnson et al, 2004; Horton et al., 2013; Petromatad, written communication, 2017).

<p>*- ⁴⁰Ar/³⁹Ar absolute ages; PALY- palynology age estimates.</p

Photomicrographs of the packstone.

<p>(A) Allochems of the packstone cemented by fine-medium sparry calcite. (B) Intact ostracod allochem. (C) Detrital feldspar. (D) Gastropod. (E) Contast showing massive micrite matrix and various-sized calcite cement of packstone. (F) Massive micrite matrix. CC-caddisfly case; Fs-feldspar; Os-ostracod; Ga-gastropod; Ca-calcite; Mi-micrite; Pl-plant fragment.</p

Thin section clast counts of the caddisfly case construction materials.

<p>Thin section clast counts of the caddisfly case construction materials.</p

Caddisfly case size variations.

<p>(A) Size distribution of the straight caddisfly cases. (B) Size distribution of the curved caddisfly cases. (1–10)- Number of caddisfly cases.</p

Local geological map overlain on the satellite imagery (modified after Zabotkin et al., 1988).

<p>Refer to the keys for map units and symbols. Fm: formation.</p

Photos of the main facies.

<p>(A) Laminated mudstone facies of the lower interval. (B) Close view of organic-rich, laminated mudstone. (C) Interbedded mudstone, tabular sandstone, siltstone, and marl facies. (D) Pillow lava of the lower interval. (E) Coarsening upward interbedded sandstone, siltstone, and mudstone facies within the upper interval. (F) Climbing ripples of the sandstone facies within the upper interval.</p

Photos of the bioherms.

<p>(A) Mound-shaped bioherm bounded by laminated, organic-rich mudstone. (B) Mound-shaped bioherm bounded by laminated mudstone. (C) Contact between bioherm and underlying mudstone. (D) Wavy laminated structure of the bioherm. (E) Tube-shaped caddisfly cases on the weathered surface of bioherm. (F) Fine laminated internal structure of the bioherm.</p