BH250-127
Title
BH250-127
Subject
Limestone with Molar Tooth Structures/Features
Description
The molar tooth structure is a distinctive feature found in fine-grained carbonate rocks, characterized by calcite-filled cracks or voids. Despite its name, it is not a fossil but a diagenetic structure composed of uniform microspar. These features likely formed early in the sedimentary history, before compaction and lithification, when calcite precipitated into sedimentary voids. Molar tooth structures are restricted to a specific interval in Earth's history, from the Mesoproterozoic to the early Neoproterozoic and are commonly associated with shallow marine carbonate environments. Their absence in modern settings has led to their use as indicators of ancient ocean chemistry and sedimentary processes.
Several hypotheses have been proposed to explain their origin, including gas expansion, seismic activity in Proterozoic basins, and calcite replacement of earlier structures. Each theory provides insight into the environmental conditions between 1.6 and 1.0 billion years ago, though no single explanation has gained full consensus.
To investigate these structures further, a geochemical study was conducted on samples from Tigray, Northern Ethiopia by my introduction to geology students (2013). Using petrographic thin sections and scanning electron microscopy, students compared the chemical composition of the molar tooth structures and surrounding host rock. Results showed the molar tooth areas contained extremely pure carbonate (99.8897 wt.% CaO), while the adjacent limestone held slightly less (approximately 95.64 wt.% CaO), supporting an early formation timing before major diagenetic change (see Table BH250-127).
Weathering affects different rock types in distinct ways, often altering their final appearance. For example, in intrusive igneous rocks, weathering leads to the formation of clay minerals, oxides, and hydroxides on the surface. These weathering products obscure the original mineral composition, making it difficult to observe the rock's primary crystalline features. In contrast, weathering of carbonate rocks tends to enhance rather than obscure internal features. It often reveals sedimentary fabrics and structures that are less visible in fresh samples.
Interestingly, weathering can improve the visibility of molar tooth structures, calcite-filled cracks or voids within fine-grained carbonate rocks. While fresh surfaces may conceal these features, weathered surfaces can expose them in much greater detail. This contrast highlights the important role surface processes play in either masking or revealing internal rock features, as exemplified by sample BH250-127 (127a and aa). In comparison, weathering typically diminishes the visibility of structures in granitic/gabbroic rocks (BH250-113B) by dulling their mineral textures and clarity. A full paper "BH250-127: The Molar Tooth Structure – A Geochemical Comparison Study", detailing the methodology and results is linked on this website.
Several hypotheses have been proposed to explain their origin, including gas expansion, seismic activity in Proterozoic basins, and calcite replacement of earlier structures. Each theory provides insight into the environmental conditions between 1.6 and 1.0 billion years ago, though no single explanation has gained full consensus.
To investigate these structures further, a geochemical study was conducted on samples from Tigray, Northern Ethiopia by my introduction to geology students (2013). Using petrographic thin sections and scanning electron microscopy, students compared the chemical composition of the molar tooth structures and surrounding host rock. Results showed the molar tooth areas contained extremely pure carbonate (99.8897 wt.% CaO), while the adjacent limestone held slightly less (approximately 95.64 wt.% CaO), supporting an early formation timing before major diagenetic change (see Table BH250-127).
Weathering affects different rock types in distinct ways, often altering their final appearance. For example, in intrusive igneous rocks, weathering leads to the formation of clay minerals, oxides, and hydroxides on the surface. These weathering products obscure the original mineral composition, making it difficult to observe the rock's primary crystalline features. In contrast, weathering of carbonate rocks tends to enhance rather than obscure internal features. It often reveals sedimentary fabrics and structures that are less visible in fresh samples.
Interestingly, weathering can improve the visibility of molar tooth structures, calcite-filled cracks or voids within fine-grained carbonate rocks. While fresh surfaces may conceal these features, weathered surfaces can expose them in much greater detail. This contrast highlights the important role surface processes play in either masking or revealing internal rock features, as exemplified by sample BH250-127 (127a and aa). In comparison, weathering typically diminishes the visibility of structures in granitic/gabbroic rocks (BH250-113B) by dulling their mineral textures and clarity. A full paper "BH250-127: The Molar Tooth Structure – A Geochemical Comparison Study", detailing the methodology and results is linked on this website.
Coverage
Location: Tigary, Ethiopia
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-127. Housed at Carleton College in Minnesota.
Contributor
Bereket Haileab
Type
Hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-127,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/149.