Sedimentary rock layers are like pages in Earth’s history book. In this assignment, you’ll act as a “geologic historian,” interpreting a stratigraphic column. After analyzing the rock sequence provided, reconstruct the geologic history of the area based on the layers.

Tell how the environment has changed over time based on the rock layers.
Discuss at least one geologic event you notice.

SOLUTION

Reconstructed Stratigraphic Column (top → bottom)

Recent soil and alluvium (thin veneer of unconsolidated silt, sand, organic material)
Yellow sandstone — well-sorted, cross-bedded (nearshore/shoreface deposits)
Dark gray shale — thin bedding, marine fossils (offshore, low-energy muds)
Fossiliferous limestone — fossil-rich, bioclastic (warm, shallow marine carbonate platform)
Coal seam (thin) — carbonaceous coal and plant fragments (coastal swamp/peat)
Reddish conglomerate — poorly sorted, rounded pebbles in sandy matrix (braided river/alluvial fan)
Basement crystalline rock (metamorphic) — older, indurated gneiss (ductile/igneous protolith)

Additionally: a vertical dike of basalt cuts through layers 6 → 2 and is overlain by the soil/alluvium.

Step-by-Step Geologic History and Environmental Changes

1. Deposition of the basement (oldest event):
The sequence begins with an old, crystalline metamorphic basement (gneiss). This represents a deep-time crustal event (tectonism and metamorphism) and is the oldest rock in the column — the stable foundation upon which later sediments were deposited.

2. Continental/fluvial deposition — conglomerate (alluvial environment):
After a period of erosion of the basement, high-energy rivers deposited the reddish conglomerate (rounded pebbles and cobbles) in alluvial channels or a braided river system. This indicates a terrestrial landscape with significant relief and sediment supply — likely a proximal, uplifted source area feeding coarse detritus into the basin.

3. Coastal swamp development — coal formation:
Conditions shifted from high-energy fluvial to low-energy coastal plain/swamp. Organic accumulation in poorly drained environments produced peat that later transformed to a thin coal seam. This indicates a rise in groundwater and reducing conditions favoring plant preservation — a transgressive tendency or stabilization in relative sea level.

4. Shallow warm seas — fossiliferous limestone (marine transgression):
Sea level rose (transgression) or subsidence increased, flooding the coastal plain. Warm, clear, shallow marine waters supported abundant carbonate-producing organisms; their skeletal debris formed the limestone. This environment represents peak marine influence and minimal terrigenous input.

5. Offshore deepening — dark gray shale (deeper marine):
Above the limestone, deposition of fine muds and shales with offshore fauna suggests deeper, quieter marine conditions — either continued transgression or basin deepening. Fine sediments settled out in low-energy offshore environments.

6. Regression/nearshore progradation — sandstone (shoreface/beach):
The overlying cross-bedded sandstone implies a later shift toward shallower, higher-energy nearshore conditions — a regression or progradation of the shoreline that deposited sand in a shoreface or deltaic setting. This indicates either a fall in sea level or increased sediment supply prograding the shoreline basinward.

7. Surface weathering and soil formation (alluvium):
Finally, after deposition of the sandstone, exposure and weathering produced soil and local alluvial deposits. Modern surficial sediments accumulated on top during recent times.

Key Geologic Event: Basalt Dike Intrusion (igneous cross-cutting event)

A basalt dike cuts vertically through the conglomerate up to the sandstone, and it is overlain by the recent soil. By the principle of cross-cutting relationships, the dike is younger than all layers it penetrates. This igneous intrusion indicates tectonic or magmatic activity after sediment deposition and before the most recent soil development. The intrusion could have caused localized contact metamorphism of adjacent sediments and provides a minimum age constraint: sediments must predate the dike.

Additional Interpretive Notes (principles used)

Superposition: Lower beds (conglomerate, basement) are older than overlying limestone, shale, and sandstone.
Original horizontality: Layers were originally deposited nearly horizontal; tilting/folding is not indicated so the sequence is likely largely undeformed.
Cross-cutting relationships: The basalt dike postdates the layers it cuts; soil/alluvium postdates the dike.
Environment change summary: Terrestrial high-energy (conglomerate) → coastal swamp (coal) → shallow carbonate sea (limestone) → deeper offshore (shale) → shoreface regression (sandstone) → exposure/soil. This sequence shows a marine transgression followed by regression, punctuated by magmatic intrusion.

How to Present This in Your Assignment

Draw and label the vertical column with the layers listed above.
Under the column, write a concise geologic history paragraph (3–6 sentences) following the timeline above.
Explain the evidence for each interpretation (e.g., fossils = marine, cross-bedding = shoreface, rounded clasts = fluvial).
Describe the basalt intrusion as the formal “geologic event” and apply cross-cutting relationships to justify its relative timing.

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