The Bear Gulch Limestone is a limestone-rich geological lens in central Montana, renowned for the quality of its marine fossils from the late Mississippian subperiod, about 324 million years ago.

The Bear Gulch Limestone reconstructs a diverse, though isolated, marine ecosystem which developed near the end of the Serpukhovian age in the Carboniferous period.

Bear Gulch fossils include a variety of fish, invertebrates, and algae occupying a number of different habitats within a preserved shallow bay.

This exceptional preservation may be a consequence of the prevailing warm monsoonal climate, as storms could frequently and rapidly cover the seabed with oxygen-poor organic runoff from shallower areas.

[2] Although up to 40 meters of sediment are found in the Bear Gulch Limestone, biostratigraphic data suggests that the lens was emplaced in only 1000 years, a geological instant.

The Central Montana Trough would have also been linked to fully marine basins on the western coastline of Laurussia,[1][14][15] but this connection may have been broken by the time of the Bear Gulch Limestone's deposition.

The only exposed portions of the sequence are found at Potter Creek Dome, a small uplifted area northeast of the Big Snowy Mountains.

[1][2] The last few limestone lenses form a large portion of the Upper Heath Formation,[14] which is sometimes termed the Bear Gulch Member in recognition of the most well-exposed and fossiliferous lens in the sequence.

[2][6][4] The creation of limestone lenses in the Heath Formation has been linked to tectonic activity extending the seaway by excavating bays out of the surrounding land.

[6][4] The final limestone deposits in the area were succeeded by freshwater lake sediments of the Cameron Creek Formation, the oldest unit of the early Pennsylvanian-age Amsden Group.

[1][2] A wide variety of biostratigraphic evidence places the Heath Formation in the upper part of the Chesterian stage, near the end of the Mississippian subperiod (early Carboniferous period).

[2] Fine-grained lithographic limestone (plattenkalk) is predominant in the Bear Gulch lens, though clay to silt-sized siliclastic material also forms a significant portion of rock in some areas.

[1][2] The thick and fossiliferous central basin facies develop along the main northwest-to-southeast axis of the bay, which is delimited by small dewatering microfaults.

Central basin sedimentation was cyclical, with thin beds of pale shale interbedded between thicker layers of dark, organic-rich limestone with a distinct oily smell.

[1][15][6] At its eastern outlet and nearby sheltered alcoves, the central basin transitions into the lightly colored laminated sediments of the Arborispongia-productid facies.

[1][14][2] The end of the bay's lifespan was indicated by a sequence of layers with leaf fragments, limestone conglomerates,[17] marls, and finally fully freshwater sediments.

Fossils of fully saltwater taxa are rare and poorly preserved relative to other environments, though organisms with a wider range of salinity tolerance (Acanthodes, gastropods, filamentous algae) are fairly common.

[1][15][2] The Bear Gulch Limestone is a konservat lagerstätte, meaning that its fossils are uniquely well-preserved, including soft tissue details which offer rare insights into the biology of Carboniferous organisms.

[2] The fine-grained sediments common in the formation allow for fossilized structures to retain fine resolution, as seen in equivalent plattenkalk-based lagerstätten throughout geological history.

Arborispongia assemblages, cephalopod shells, and large vertebrate bones occasionally project through several thin layers, approaching three-dimensional preservation.

Rare disarticulated fragments may correspond to large or buoyant carcasses which rise to the water surface to gradually decay and fall apart in a "bloat and float" taphonomic process.

[14][2] Fossils are dispersed throughout the Bear Gulch lens, rather than concentrated in specific fossil-rich beds (which would be expected if organisms were killed by algal blooms).

[4] Many indicators of paleoecology and paleobiology have been preserved, from associations between encrusting shelled organisms and algae,[23] to gut contents and other feeding traces,[13] and even some animals fossilized while mating.

The Bear Gulch Limestone is renowned for its unusual and ecologically diverse chondrichthyans, the group of cartilaginous fish containing modern sharks, rays, and chimaeras.