Last night, just for the heck of it, I looked up the expression contralto on Wikipedia and found out that it was the lowest singing voice type for women when compared to the higher soprano and mezzo-soprano voices. According to this YouTube video, singers such as Cher and Annie Lennox have the contralto voice. I think that when it comes to Japanese popular singers, Akina Nakamori（中森明菜）especially after her first few aidoru years and Kahoru Kohiruimaki（小比類巻かほる）pop up in my head. Perhaps Mariya Takeuchi（竹内まりや）is a mezzo-soprano?
Junko Mihara（三原順子）was a singer that I would also classify as having a voice which is pretty deep. That was certainly the case when I first heard her perform "Honki de Love Me Good"（ホンキでLove Me Good) on the 1982 Kohaku Utagassen. And some further proof in the pudding was provided from this song, her 4th single from May 1981, "Sunnyside Connection".
In fact, listening to Mihara's contralto, I thought of the aforementioned Cher although I realize that this was never a type of song that Cher had ever tackled. Actually "Sunnyside Connection" struck me in terms of the arrangement as a driving song that Akina Nakamori would tackle early in her career. That wailing electric guitar and that piping synthesizer got me rather nostalgic. Machiko Ryu（竜真知子）provided the words while Kazuya Amikura（網倉一也）took care of the music for this single that peaked at No. 14.
As for Mihara right now, she's still a member of the Japanese Parliament.