Somerset Dam

 ....all the fountains of the great deep broken up, and the windows of heaven were opened.

Genesis 7:11

 

"The Somerset Dam layered mafic intrusion is situated immediately

west of the village of Somerset Dam at 152°32’E and 27°7’S, some 65km northwest of the city of Brisbane in south-east Queensland
on Australia’s east coast.

 

Together with other acid to intermediate volcanics and volcaniclastic and clastic sediments, the Neara Volcanic were deposited in the downfaulted Esk Trough, a Mesozoic basin within the Paleozoic New England Orogen, elongated blocks of which are exposed to the west and east paralleling the sides of the Esk Trough (Flood & Aitchison, 1993; Harrington & Korsch, 1985). 

To the south, the sediments and volcanics of the Esk Trough plunge beneath the clastic sediments and coal seams of the Upper Triassic–Jurassic Moreton Basin. The region is characterized by younger basaltic volcanism, radioisotope dated as Late Oligocene to Early Miocene (Day et al, 1983; Webb, Stevens, & McDougall, 1967). These volcanic rocks form a continuous belt extending north-northwest from near the coast south of Brisbane for over 200km parallel and close to the mountains west of the Brisbane River Valley, which has been eroded into the sediments and volcanics of the Esk Trough. This volcanism produced large,now eroded,shield volcanoes,complex lavafields,ring complexes and localized cones, plugs, laccoliths, sills and dikes (Ewart & Grenfell, 1985). 

Other Tertiary volcanic remnants are found outcropping to the east and northeast of the Somerset Dam intrusion, perhaps suggesting a wider distribution of these lava flows across the region prior to the erosion.

 

However, an interpretation of the regional geologic record within a

Biblical framework and timescale would place the deposition of the volcanics and sediments in these Mesozoic basins as late in the
Flood year, with the subsequent erosion at the end of the Flood and extending into the post-Flood era to the present day. Thus if the Tertiary basaltic volcanic activity occurred early in the post-Flood era, then it would not be far removed in time, or spatially, from
the Somerset Dam gabbro intrusion that has been interpreted as the remains of the magma chamber feeding a basaltic volcano.

  

Within this gabbro intrusion there is an exposed sequence of 22 saucer-shaped macrolayers, 3–50m thick generally dipping inwards at 10–20°. The contacts between these prominent layers are sharply defined, generally to within 10cm, and are phase, modal mineralogy and textural contacts. These macrolayers appear to be stratigraphically and laterally homogeneous. The repetition of these
macrolayers has allowed the recognition of at least six well developed cyclic units, ranging from 30 to 150m thick (average about 80m thick).  

The macrolayers are limited to only four main rock types, which are defined in terms of their essential cumulus mineral phases (distinguished texturally from the intercumulus mineral phases) (Irvine, 1982; Wager, Brown, & Wadsworth, 1960). 

---These four rock types constituting the macrolayers are

leucogabbro or anorthosite (plagioclase cumulate), troctolite (plagioclase + olivine cumulate), olivine gabbro (plagioclase + augite + olivine cumulate), and oxide (or ferri-) gabbro (plagioclase + augite ± olivine + magnetite + ilmenite cumulate).
The definition of these cyclic units,selecting which of these macrolayers commences each cycle, is somewhat interpretive. 

The choice is strongly influenced by what is expected to be the order of crystallization and the magma fractionation pattern, because there
is commonly no clear field evidence to identify the base and top of a cyclic unit. In the Somerset Dam gabbro intrusion, Mathison (1964, 1967, 1970, 1975) chose to define each cyclic unit to be the sequence
troctolite–olivine gabbro–oxide gabbro–leucogabbro, because troctolite was considered the least fractionated rock type, and cryptic trends generally suggesteda reversal at the bases of the troctolites.

 

---The distribution of the major element oxides between the four different rock types in the macrolayers of each cyclic unit follows the pattern in the average whole-rock major element oxide compositions for these rocks already reported by Mathison (1967, 1987). The
oxide gabbros have the lowest SiO2 and highest Fe2O3 contents, and the anorthosites (leucogabbros) have the highest SiO2 and lowest Fe2O3 contents. The Al2O3 content is highest in anorthosite at the base of each cyclic unit and decreases upwards through troctolite
and olivine gabbro to the lowest content in the oxide gabbro, which parallels the decreasing plagioclase content (Mathison, 1967) found that the compositional changes with increasing height and increasing fractionation in a typical cyclic unit were clearly evident from the
major element oxide data, ...generally involve increases in total iron (bothFe2+ and Fe3+), the Fe3+/Fe2+ratio, TiO2 and MnO, and
decreases in Al2O3, SiO2, Na2O, and K2O.

 

Furthermore,
as expected, the pattern of variations in each cyclic unit of the major and trace elements parallels the changes in mineralogy. Thus the P2O5 content parallels the presence of accessory apatite, Sr parallels the abundance of plagioclase, and rare earth elements such as La, Ce, and Nd are often slightly more concentrated in the oxide gabbros. 

 

The initial 40Ar* is 0.00029ppm, which is a very small quantity of excess radiogenic 40Ar in violation of the assumption of zero 40Ar in the model age technique. This K-Ar isochron “age” is significantly discordant with the three sets of published K-Ar determinations on hornblende and plagioclase of Webb & McDougall (1967), recalculated by Murphy et al (1976; 1987) using the IUGS constants of Dalrymple (1979), of 218 and 220Ma, and 212 and 213Ma respectively, which yield an “averaged age” of 216±4Ma. Similarly, this 15-point K-Ar isochron “age” of 174±8Ma is strongly discordant with the “pooled apparent age” of 224.2±4.8Ma based
on 40Ar/39Ar determinations on three K-feldspar, four muscovite, one hornblende and four biotite grains by Walker (1998).

 

Conclusions

Mineralogical, geochemical, and isotopic evidences all indicate that the cyclic units of gabbro macrolayers in the Somerset Dam layered mafic intrusion are coeval and were derived from the same parental basaltic magma. The original endowment of Ar,
Sr, Nd, and Pb isotopes was homogeneously mixed, a necessary condition for successful conventional radioisotopic dating, particularly by the isochron method.  

However, the four analyzed radioisotope systems yield discordant whole-rock isochron “ages.” The Sr, Nd, and Pb isotopes also reveal that the parental basaltic magma of the Somerset Dam layered gabbro intrusion was partially melted from a depleted
mantle source in a single episode, and that it suffered negligible crustal contamination as it ascended up and into the sub-volcanic magma chamber. 

Nevertheless, it is concluded that the Somerset Dam layered mafic intrusion has inherited the radioisotopic signature of its mantle source, and thus its present radioisotopic ratios do not provide its true age by the conventional radioisotopic dating techniques."

ICR