Reference

The transition between the Pottsville Formation and the Allegheny Formations is a significant geological boundary that occurs within the Pennsylvanian period. Here are the key aspects of this transition:

1. Stratigraphy:

   • The Pottsville Formation predominantly consists of sandstones, conglomerates, siltstones, and shales. It is often characterized by its coarse-grained nature and contains significant coal beds.
   • The Allegheny Formation overlays the Pottsville Formation and is marked by more diverse lithological composition, including finer-grained shales, siltstones, sandstones, claystones, limestones, and multiple coal seams.

2. Depositional Environments:

   • The Pottsville Formation was deposited in a more dynamic fluvial environment with high energy conditions leading to the deposition of conglomerates and coarse-grained sandstones.
   • Transitioning into the Allegheny Formation, the environment shifted towards varied depositional conditions including fluvial, deltaic, and shallow marine settings. These conditions allowed for the deposition of finer sediments and the formation of persistent coal beds.

3. Geological Boundaries:

   • The contact between these formations is recognized as a change from the massive, often pebbly sandstones of the Pottsville to the finer and more distinctly bedded strata of the Allegheny. This change is marked by a transition in both grain size and sedimentary structures.
   • In some regions, a notable unconformity or a disconformity may occur between these formations, highlighting a break or shift in depositional regimes.

4. Regional Variations:

   • The nature of transition and the characteristics of these formations can vary regionally due to the differences in paleogeographic and tectonic settings at the time of deposition. For instance, the depositional patterns in the Appalachian Basin may differ from those in the Illinois Basin.

Visual Reference:

Here are some visual aids that may help understand the transition:

Generalized stratigraphic cross-section

Idealized depositional model

Source References:

• US Geological Survey report on Pottsville and Allegheny formations
• Wikipedia entry on Pottsville Formation

These insights provide a foundational understanding of how the Pottsville Formation transitions into the Allegheny Formations. If you need more specific details, examining local geological studies or stratigraphic columns can provide more precise information for particular regions.