Hiatus
A hiatus, in stratigraphy and petroleum geology, is a gap in the sedimentary record representing a period of time during which sediment was either not deposited (a non-depositional hiatus, caused by sea level rise that starved the location of sediment, by uplift that placed the area above base level, or by changes in current patterns that redirected sediment transport away from the location) or was deposited and subsequently removed by erosion (an erosional hiatus, caused by subaerial exposure and weathering, submarine erosion, or tectonic uplift and denudation), leaving adjacent sedimentary units in contact across a boundary that represents a time interval significantly longer than would be expected from simple calculation of deposition rates; a hiatus is the temporal expression of what appears geometrically as an unconformity (a surface separating rock masses of distinctly different ages) and is quantified by the duration of time represented by the gap, which can range from a few thousand years (in a sequence stratigraphy context, a brief non-depositional interval at a sequence boundary) to hundreds of millions of years (in a major unconformity such as the base of the Cambrian transgression over Precambrian basement); in petroleum geology, hiatuses are commercially significant because they mark surfaces across which reservoir, source, and seal relationships may be disrupted, where trap configurations may be defined by truncation of reservoir against the unconformity surface, and where diagenetic processes concentrated at the exposure surface (laterite, caliche, karst) may either enhance or destroy reservoir quality in adjacent rock.
Key Takeaways
- The biostratigraphic calibration of hiatus duration uses the fossil record to determine the age of sediments immediately below and immediately above the unconformity surface, with the difference in age defining the duration of the hiatus; biostratigraphic dating uses index fossils (species with short stratigraphic ranges that define specific time intervals), nannofossil zones (calcareous nannoplankton with excellent stratigraphic resolution in marine sediments), and foraminiferal biozones to assign ages to samples from just below and just above the unconformity contact; the duration of the hiatus is then calculated as the difference between the age of the youngest fossils below the unconformity and the oldest fossils above it; in well calibration, the hiatus is often detectable as an anomalous absence of expected biozone markers at the unconformity depth, with the biozone immediately below the gap being older than expected given the depth below the biozone immediately above the gap; quantifying the hiatus duration is important for reconstructing the thermal maturity history of the source rock (a long hiatus with erosion may have reduced the burial depth at which source rock generation occurred, affecting the timing of petroleum generation relative to trap formation).
- Subaerial exposure at a hiatus surface creates diagenetic products that significantly alter reservoir quality in carbonates: during the exposure period, meteoric water infiltrates the exposed carbonate rock, dissolving calcium carbonate along preferential pathways (fractures, grain boundaries, interparticle pores) to create secondary porosity as vugs, moldic pores, and dissolution cavities; this dissolution porosity can dramatically enhance reservoir quality in carbonates that had low primary porosity (tight packstones and wackestones become high-porosity vuggy carbonates after exposure and dissolution); conversely, the same meteoric water may precipitate calcite cement in other locations (as mineral-saturated groundwater loses CO2 and becomes supersaturated with respect to calcite), creating tight, cemented zones adjacent to the dissolution porosity that creates heterogeneous reservoir quality distribution; the karst-related porosity of the Ellenburger dolomite in Texas, the Arbuckle dolomite in Kansas, and the Paleozoic carbonate reservoirs of the Tarim Basin in China are all examples of hiatus-related dissolution porosity that forms one of the most prolific reservoir types in the geological record.
- Unconformity traps exploit the stratigraphic relationship between reservoir rocks that were truncated by erosion at the unconformity surface and are now sealed by the overlying impermeable rock that was deposited after the hiatus: in a classic truncation trap, a porous reservoir sandstone was tilted, uplifted, and eroded at the unconformity surface, with the erosional truncation creating the updip seal; when subsequent sea-level rise buried the eroded surface under an impermeable mudstone or shale, the geometry creates a lateral seal at the unconformity surface and an overlying vertical seal from the cap rock; hydrocarbons migrating updip along the reservoir dip are trapped at the unconformity where the reservoir has been cut off; the East Texas field (the largest oil field ever discovered in the contiguous United States at the time of its discovery in 1930) is a classic truncation trap against the Sabine Uplift unconformity, where the Woodbine sandstone reservoir is sealed updip by the erosional truncation against the Austin Chalk; identifying and mapping unconformity truncation traps requires high-quality seismic imaging of the reflector terminations against the unconformity surface and well control that documents the age of the unconformity.
- Sequence stratigraphic hiatuses within sedimentary cycles are shorter and less dramatic than the major unconformities that dominate structural geology discussions, but they are pervasive throughout the sedimentary record and their positions mark the most important surfaces for reservoir-seal-trap analysis: the sequence boundary hiatus (at the base of the lowstand systems tract) represents the time of maximum sea level fall and often corresponds to the surface of maximum subaerial exposure and fluvial incision of the shelf, below which the reservoir quality may be enhanced by meteoric diagenesis and above which the first lowstand fan turbidites are deposited; the maximum flooding surface hiatus (at the top of the transgressive systems tract, corresponding to the deepest water and maximum sediment starvation) marks the position of the most condensed section (the thinnest interval representing the longest time, often rich in organic matter from the slow organic-matter preservation in quiet, deep water), which may be an excellent source rock or radioactive shale marker used for basin-wide correlation; recognizing these within-sequence hiatuses from well logs (gamma ray spikes at the condensed section, sonic velocity reversals at the sequence boundary) and seismic (reflector terminations, onlap, truncation patterns) is the operational skill of sequence stratigraphy applied to petroleum basin analysis.
- Hiatus recognition in well logs uses indirect indicators rather than direct observation because the gap in the sedimentary record by definition contains no rock to sample or log: the most reliable log indicators of a hiatus are abrupt gamma ray increases (from the condensed section organic-rich sediments that often overlie a sequence boundary), anomalous sonic velocity reversals (a sudden velocity increase from cementation or diagenetic alteration at the erosion surface, followed by a velocity decrease as normal burial compaction is restored), formation water chemistry changes across the contact (measured from resistivity log response calibrated to regional formation water maps), or discontinuities in the pressure gradient measured from formation pressure tests (indicating that the rock above and below the unconformity are in different pressure compartments, sealed by the unconformity surface itself); in cases where the hiatus represents only a few thousand years (within the resolution of biostratigraphic dating), the hiatus may be invisible in the log response and identifiable only from the biostratigraphic absence of expected fossil zones.
Fast Facts
The term "hiatus" in its geological sense was popularized by the American geologist Grove Karl Gilbert in the late 19th century, who used it to describe the time gap represented by an unconformity in his studies of the Henry Mountains of Utah. Gilbert recognized that unconformities were not just surfaces of missing rock but records of time — the time during which the earth's surface was exposed, eroded, or simply not receiving sediment. This insight, that geology records time as well as space and that some time intervals leave no geological record at all, is one of the foundational concepts of historical geology and is directly applied in modern sequence stratigraphy, which uses the pattern and duration of hiatuses to reconstruct the sea-level history of sedimentary basins.
What Is a Hiatus?
A hiatus is missing time in the rock record. Where you expect to find rock deposited continuously over millions of years, you find instead a surface where one rock sits on another and the clocks of sedimentation show a gap of thousands or millions of years between them. The rock that should have been deposited during that gap either never existed (the environment was above sea level or beyond the reach of sediment supply during the gap) or existed and was removed by erosion (the land was uplifted, exposed, and the accumulated sediments were stripped away by rivers, glaciers, or the sea). Either way, the gap is invisible to a geologist looking at the outcrop or the core without biostratigraphic tools: the rock below looks like rock and the rock above looks like rock and only the fossils reveal that something is missing in between. In petroleum geology, hiatuses matter because they coincide with some of the most important surfaces for petroleum system analysis: the unconformity surfaces where truncated reservoir rocks are sealed updip, where diagenetic porosity enhancement by meteoric waters created the karst reservoirs that produce billions of barrels, and where source rock condensed sections rich in organic matter were deposited in the quiet, oxygen-depleted waters of maximum flooding events that preceded the hiatus.
Synonyms and Related Terminology
Hiatus is synonymous with stratigraphic gap or time gap. The rock surface that bounds the hiatus is called an unconformity or a disconformity (when the beds above and below are parallel). Related terms include unconformity (the rock surface that bounds a hiatus, separating rock masses of distinctly different ages due to an intervening period of non-deposition or erosion, recognized in outcrops and seismic sections as a surface with reflector terminations (truncation below, onlap above) that defines a major stratigraphic break in the sedimentary record), sequence boundary (in sequence stratigraphy, the unconformity surface (or its correlative conformity in basin centers) that marks the base of a depositional sequence, corresponding to the time of maximum sea level fall and maximum subaerial exposure, above which the lowstand systems tract records the first renewed deposition after the hiatus), condensed section (the thin, organic-rich, often radioactive sedimentary interval deposited during maximum flooding when sediment input was minimal and the water column deep and oxygen-depleted, frequently associated with the maximum flooding surface that is the stratigraphically highest expression of the hiatus-bounded transgressive systems tract), truncation trap (a petroleum trap formed where a tilted reservoir unit has been eroded at an unconformity surface and subsequently sealed by the overlying impermeable cap rock deposited after the hiatus, with the erosional termination of the reservoir providing the updip seal that concentrates hydrocarbons in the remaining portion of the reservoir below the unconformity), and karst (the landscape and subsurface porosity system created by dissolution of carbonate rock by meteoric water during a period of subaerial exposure at a hiatus surface, creating vuggy, moldic, and cavernous porosity that forms some of the most productive petroleum reservoir types in carbonates worldwide).