Questions: Sediment Provenance: Using Detrital Minerals to Trace Source

Question 1

A sandstone contains abundant olivine and pyroxene grains. What does this mineral assemblage most likely indicate about the sediment's source and transport history?

• A. A mature, recycled sediment that survived multiple weathering cycles from a distant granitic source
• B. A young, proximal source such as a volcano or active orogenic belt with minimal transport and weathering
• C. A deep marine setting where chemical weathering is suppressed by cold temperatures
• D. An ancient cratonic source where only the most stable minerals remain

Answer: (B)

Olivine and pyroxene are chemically unstable at surface conditions and break down quickly during weathering and transport. Their presence in a sandstone indicates that the source was nearby and erosion was rapid — there wasn't enough time for these minerals to weather away. Active volcanic arcs, rifted margins, and thrust belts where uplift outpaces weathering are classic settings. A distant or old source would leave only quartz, zircon, and other resistant minerals. Abundant unstable minerals = young, hot, proximal source.

Question 2

A geologist finds chrome spinel grains in a sandstone. Chrome spinel is nearly diagnostic of what type of source rock?

• A. Granitic continental basement, because chromium is concentrated in felsic melts
• B. High-grade metamorphic rocks like eclogite or granulite
• C. Carbonate platform sediments, which sequester chromium in organic matter
• D. Ultramafic rocks such as oceanic mantle peridotite or ophiolite sequences

Answer: (D)

Chrome spinel forms in mantle-derived ultramafic rocks and is stable enough to survive transport and diagenesis. Its presence in a sandstone is nearly unique evidence that the source terrane included ultramafic material — exposed oceanic mantle, ophiolite obducted onto a continent, or layered intrusions. Granites and their derivatives contain almost no chrome spinel. This is why heavy mineral analysis can fingerprint specific source terranes far more precisely than bulk mineralogy alone: each heavy mineral has a restricted paragenesis that points to specific rock types.

Question 3

The presence of abundant olivine grains in a sandstone indicates that the sediment traveled a long distance from its source, because mainly long-transport processes can concentrate dense minerals like olivine.

• T. True
• F. False

Answer: False

This reverses the logic of differential mineral stability. Olivine is chemically unstable and weathers quickly. Long transport = prolonged exposure to weathering = olivine destroyed. Abundant olivine therefore indicates SHORT transport distance and rapid burial — the grains didn't have time to weather away. The stability hierarchy runs from most resistant (quartz, zircon) to least resistant (olivine, pyroxene). High-stability minerals survive long transport; low-stability minerals indicate proximity to the source.

Question 4

Two sandstones with identical QFR compositions could still have different provenances if they experienced different intensities of weathering and transport before deposition.

• T. True
• F. False

Answer: True

Weathering and transport intensity progressively remove unstable minerals and enrich surviving grains in stable ones (mainly quartz). A granite source can produce an arkose (feldspar-rich) in an arid, short-transport setting, or a quartz arenite (quartz-rich) if the same granite is exposed to intense tropical weathering or repeated recycling. Identical QFR plots can result from very different source rocks if the weathering and recycling histories are correspondingly different. This is why heavy mineral suites and isotopic fingerprinting (e.g., U-Pb ages on detrital zircon) are used to resolve provenance ambiguity that bulk QFR cannot distinguish.

Question 5

Why does a feldspar-rich sandstone (arkose) point to a different tectonic setting and transport history than a quartz-rich sandstone, even if both could have been derived from granitic source rocks?

Model answer: Feldspar is moderately unstable — it weathers into clay minerals under humid conditions over time. For feldspar to survive in abundance, chemical weathering must be limited (arid climate, cold temperatures) and transport must be short and rapid (so grains reach burial before weathering destroys them). This combination is characteristic of fault-bounded basins receiving coarse sediment directly from an adjacent granite range, like the Basin and Range province. A quartz-rich sandstone from the same granite source requires either intense weathering that destroyed all feldspar, or multiple recycling cycles through older sedimentary rocks. Same source rock, opposite extremes of weathering/transport intensity.

The key insight is that the QFR composition reflects not just source rock type but the integrated weathering and transport history. Mineral assemblages are a palimpsest: the source signal is modified by every step of the sedimentary cycle. Interpreting provenance requires reasoning about what was present in the source AND what survived the journey — the difference between the two tells you about climate, relief, and basin geometry.