Ma (Mega Annum): Geologic Time Notation, WCSB Formation Ages, and Radiometric Dating
Ma is the standard scientific abbreviation for "mega annum," meaning one million years, and it is the unit geoscientists use most often to state the age of rocks, the timing of geologic events, and the position of a formation within the global time scale. The term comes from the SI prefix "mega" (10 to the sixth power) attached to "annum," the Latin word for year, so 1 Ma equals 1,000,000 years. In practice, Ma is used two ways that are worth keeping straight. When it describes an age or a point in the past it means "millions of years before present," so the Frasnian stage of the Late Devonian, which hosts the Leduc and Nisku carbonate reefs of the Western Canadian Sedimentary Basin, is dated at roughly 372 to 382 Ma. When it describes a duration it means "millions of years" of elapsed time, and some authors write "Myr" or "m.y." to signal duration rather than a fixed date. A related abbreviation, Ga (giga annum, one billion years), covers Precambrian and planetary time, while ka (kilo annum, one thousand years) covers recent glacial and archaeological time. Under the international convention adopted by the International Commission on Stratigraphy and the IUGS, "present" is fixed at the year 1950 for consistency, though for events tens of millions of years old that offset is negligible. For a working geologist in Alberta or British Columbia, Ma is the currency of the mud log, the biostratigraphic report, and the completions plan, because knowing that the Montney was deposited in the Early Triassic near 250 Ma, or that the McMurray oil sands host rock is Early Cretaceous near 112 to 100 Ma, tells the operator which source rocks charged the reservoir, what diagenetic history the pore system endured, and which analogue fields to compare against. Ages expressed in Ma are anchored by radiometric dating of minerals such as zircon, which trap uranium that decays to lead at a known rate, and by biostratigraphy, which correlates fossil assemblages such as conodonts, ammonites, and palynomorphs to the numeric time scale. Every AER or provincial stratigraphic chart converts these methods into a single Ma value per boundary, and those boundaries are what let a geologist tie a well in the Deep Basin to a well 400 km away in exactly the same time interval.
Key Takeaways
- Ma Means One Million Years: The abbreviation combines the SI prefix "mega" (10^6) with the Latin "annum" (year), so 1 Ma equals 1,000,000 years. Used as an age it means millions of years before present; used as a span it means millions of years of duration. The companion units are Ga (10^9 years, one billion) for Precambrian and planetary time and ka (10^3 years, one thousand) for glacial and recent time.
- Present Is Fixed At 1950: By international stratigraphic convention the "before present" datum is anchored to the calendar year 1950, matching the radiocarbon reference year. For deep-time ages measured in tens or hundreds of Ma the 76-year offset is far smaller than the dating uncertainty, so it is ignored, but it matters when Ma and ka scales are stitched together across the Quaternary.
- WCSB Formation Ages In Ma: Key Western Canadian intervals carry firm ages: Leduc and Nisku reefs at roughly 372 to 382 Ma (Late Devonian), Montney at about 247 to 251 Ma (Early Triassic), Cardium near 90 Ma (Turonian), and the McMurray oil sands at roughly 100 to 112 Ma (Early Cretaceous). These numbers set source-rock timing and thermal maturity expectations.
- Anchored By Radiometric Dating: Numeric Ma boundaries come from isotope systems such as uranium-lead in zircon, potassium-argon, and argon-argon, each with a known decay half-life. Zircon U-Pb ages routinely resolve stage boundaries to better than 1 Ma, and volcanic ash beds (bentonites) interbedded in WCSB shales give datable tie points that calibrate the biostratigraphic column.
- The Basis Of Correlation: Because every stratigraphic boundary is assigned one Ma value, geologists can correlate a well in the BC Deep Basin to a well in southeast Saskatchewan by matching time intervals rather than just lithology. This time framework underpins play mapping, reserve booking, and the choice of production analogues across hundreds of kilometres.
Reading Ma On A WCSB Stratigraphic Chart
On an AER or Alberta Geological Survey stratigraphic chart, the left axis is calibrated in Ma and each formation top plots against its depositional age. A geologist logging a Deep Basin well sees the Cretaceous Cardium at roughly 90 Ma sitting far above the Triassic Montney near 250 Ma, with a Jurassic section and multiple unconformities between them representing tens of Ma of missing or eroded time. Those gaps, called hiatuses, are read directly off the Ma scale: a boundary jump from 145 Ma to 100 Ma flags a 45 Ma unconformity. Knowing the numeric age also fixes the burial and heating history that turned kerogen into oil or gas, which is why the Ma value is more than a label.
Ma Versus Myr And The Duration Problem
Confusion arises because Ma is sometimes used for a point in time and sometimes for a length of time. Best practice, endorsed by the North American Stratigraphic Code, reserves Ma for absolute ages before present and uses Myr (or "m.y.") for durations. Saying the Montney accumulated "over roughly 6 Myr" describes a span, while saying it began "at about 251 Ma" fixes a date. In reservoir work the distinction is practical: source-rock maturation models integrate temperature over a duration in Myr, whereas seismic and biostratigraphic tie points are quoted as ages in Ma. Mixing the two produces errors in basin-modelling software that can misstate hydrocarbon generation timing.
Fast Facts
The 1977 recommendation by the International Union of Geological Sciences to standardize "Ma," "Ga," and "ka" replaced a chaotic mix of "m.y.B.P.," "AE" (aeon), and "b.y." that had cluttered mid-century literature. The choice of 1950 as the "present" datum was borrowed directly from radiocarbon dating, where 1950 marks the pre-atomic-testing baseline before bomb-derived carbon-14 contaminated the atmosphere, so a convention born in archaeology now silently anchors the age printed on every petroleum stratigraphic chart in Canada.
Related Terms
Ma is the numeric backbone of several connected concepts. Stratigraphy orders rock layers in time, and every boundary it defines is assigned a value in Ma. Biostratigraphy uses fossil ranges such as conodonts and palynomorphs to place a rock at its correct Ma position when no datable mineral is present. Unconformity surfaces represent missing Ma between two formations, a gap read straight off the time scale. And source rock maturation depends on how many Myr a kerogen-rich shale spent inside the oil window, tying elapsed time directly to Ma-dated burial history.
Real-World WCSB Scenario: Dating A Deep Basin Ash Bed
An operator drilling a Montney horizontal near Dawson Creek, British Columbia recovered a thin bentonite (volcanic ash) marker within the reservoir section. A commercial lab ran uranium-lead dating on zircon crystals separated from the ash and returned an age of 249.3 Ma, plus or minus 0.4 Ma, placing the bed firmly in the Early Triassic. That single number, costing roughly 4,000 to 6,000 CAD for the analysis, let the geoscience team tie the local reservoir subdivision to regional Montney chronostratigraphy across the BC-Alberta border and to published thermal-maturity models.
The confirmed age let the team correlate their landing zone to analogue wells with known estimated ultimate recovery, adjust the completions spacing to match the proven interval, and defend the reserve booking to auditors. What began as a 2 cm ash layer became a precise Ma anchor that de-risked a multi-well pad program worth tens of millions of CAD.